Daylight responsive led illumination panel with color temperature control

ABSTRACT

An illumination panel employing two or more groups of light emitting diodes (LEDs) configured to emit light in different color temperatures, an optically transmissive sheet with light deflecting surface structures, and an LED driver electrically connected to the LEDs. The illumination panel further incorporates two or more LED intensity control channels and one or more pulse width modulation (PWM) circuits. The LED intensity control channels are configured to independently control the light output from the individual LED groups and the LED driver is further configured to provide dimming of the LEDs in response to a change of the intensity of incident natural daylight.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 17/128,105filed on Dec. 19, 2020, which is a continuation of application Ser. No.15/481,386 filed on Apr. 6, 2017, which is a continuation of applicationSer. No. 13/682,004 filed on Nov. 20, 2012. This application also claimspriority from U.S. provisional application Ser. No. 61/563,018 filed onNov. 22, 2011, incorporated herein by reference in its entirety, andfrom U.S. provisional application Ser. No. 61/648,236 filed on May 17,2012, incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not Applicable

NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

A portion of the material in this patent document is subject tocopyright protection under the copyright laws of the United States andof other countries. The owner of the copyright rights has no objectionto the facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the United States Patent andTrademark Office publicly available file or records, but otherwisereserves all copyright rights whatsoever. The copyright owner does nothereby waive any of its rights to have this patent document maintainedin secrecy, including without limitation its rights pursuant to 37C.F.R. § 1.14.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to light emitting waveguides such lightpipes, optical fibers or planar waveguides provided with a series oflight-deflecting features distributed along the optical path andconfigured to create a controlled illumination pattern emitted from amajor waveguide's surface. This invention also relates to opticalilluminators and light distribution systems employing such waveguides,for example, panel luminaires, side-emitting optical fibers, edge-litLED front lights and backlights, lighting panels, LCD displaybacklights, daylighting luminaires, diffusers, computer screens,advertising displays, road signs, and the like, as well as to a methodfor redistributing light from various-type light sources.

2. Description of Background Art

Conventionally, light emitting devices employing a waveguide include aseries of optical features distributed along the light propagation pathin the waveguide and configured to extract light from the waveguide in aperpendicular direction. The optical features are conventionally formedby small cuts, notches or grooves in the waveguide surface which extractlight by means of reflection, refraction and/or scattering.

FIG. 1 depicts an example of the prior-art planar slab waveguide whichemploys an array of surface microstructures formed in a major surface ofthe waveguide. FIG. 2 depicts a prior-art planar slab waveguide havingan array of linear V-grooves formed in the waveguide's surface. FIG. 3depicts an example of the conventional side-emitting optical fiber whichincludes a series of notches formed in the fiber's surface along itslongitudinal axis.

FIG. 4 generalizes the optical operation of the prior-art devices shownin FIG. 1 , FIG. 2 and FIG. 3 and depicts a cross-section of thewaveguide and an individual light extracting feature formed in thewaveguide's surface. The light extracting feature commonly has thecross-sectional shape of a prismatic cavity and employs a reflectiveface inclined at a considerable slope angle to the surface of thewaveguide, usually about 45° or so. Since the light is confined withinthe waveguide by means of a total internal reflection (TIR) fromwaveguide's longitudinal walls, this slope angle generally needs to besufficiently high in order to overcome TIR and allow the reflected lightto exit from the light emitting surface opposing the light extractingfeature. As illustrated FIG. 4 , at least some light rays may bereflected from the reflective face of the light extracting feature andthen exit from the opposing surface of the waveguide. Accordingly, whena large array of such light extracting feature is distributed along amajor surface of the waveguide, the waveguide can emit luminous fluxfrom the opposing surface in a broad angular range.

The reflective face usually has an optically transmissive surface andprovides reflection by means of TIR when the incidence angles aregreater than the TIR angle with respect to a normal to the reflectiveface. However, since the light rays propagating in the waveguide haveessentially random angular distribution within the acceptance angle ofthe waveguide, at least a portion of the rays can strike the reflectiveface at angles being lower than the TIR angle. In this case, TIR willnot occur and the respective rays may exit from the waveguide throughthe unwanted face (see FIG. 4 ), resulting in light loss and reducedsystem efficiency. The reflective faces can be selectively mirrored toeliminate this light spillage. However, doing so will introducereflection losses at the mirrored surface compared to the lossless TIRand will also add fabrication steps, such as fabricating a mask,applying the mask to the waveguide surface with precision alignment,vacuum metallization, etc.

In conventional edge-lit waveguide illumination devices, the lightextracted from the waveguide by the sloped reflective faces generallyhas a high angular dispersion from the waveguide's surface normal.Particularly, the divergence of light emitted by prior-artwaveguide-based devices often approximates that of a lam bertian sourcewith a full 180° angular spread. The lack of beam directionality hampersthe utility of conventional waveguide illumination systems in theapplications requiring at least some degree of light collimation.

Furthermore, the sloped reflective surfaces of light extracting featuresrefract light propagating along the line of sight perpendicular to thewaveguide's surface. This makes the conventional devices ill suited forthe front light applications in which an edge-lit lighting panel ispositioned in front of a viewable screen or image print. Each of thelight extracting features alters the light path from the viewer to theprint and bends the light towards other portions of the print comparedto the neighboring smooth areas of the front light panel. As a result,the visual appearance and resolution of the print may deteriorate.Considering than at least some light propagating in the waveguide mayalso escape from the waveguide toward the viewer, the print contrast mayalso be affected.

Also, when the front and rear surfaces of the planar waveguide generallyhave the same optical properties, e.g., being characterized by the samestepped drop in the refractive index, the light ray which obtains anon-TIR propagation angle within the waveguide may escape from eithersurface. Various mechanisms may contribute to such light leakage. Therays which propagate at less-than-TIR angles (with respect to surfacenormal) may include high-incidence-angle portions of the initial lightbeam injected at the waveguide's edge, light scattered by impurities inthe waveguide, stray light from light extraction elements, stray lightresulting from natural divergence or leakage from the waveguide, lightwhich propagation angles are altered by the non-parallelism of waveguidewalls, light reflected from the opposing wall by means of a Fresnelreflection, etc. Since such light has about equal chance to escapethrough either front or rear surface, at least a substantial portion ofit will exit from the unwanted side of the waveguide resulting in energyloss and considerable glare. The prior-art lighting panels employinglight extraction features based on light scattering rather than onreflection typically introduce even more unwanted glare and lightspillage due to the uncontrolled nature of light scattering mechanism.Additionally, such lighting panels are usually characterized by arelatively high level of opacity and either substantially opaque or canbe translucent at best, but not fully transparent, which inhibits thebasic light guiding function of the panel. Yet further, when theconventional planar waveguide employing light extracting features (suchas surface microstructure or scattering elements) is lit from an edge,at least a portion of light extracted by these features is emittedtowards the viewer which substantially degrades the contrast andvisibility of the bodies or images disposed behind the waveguide. Thisprevents using these panels for front lights where the perceptiblequality of the background to be lit is important.

Besides sometimes being characterized by reduced optical qualities orlight spillage, the conventional systems employing relatively deep cuts,notches or grooves may also be affected by at least some loss ofstructural strength and rigidity compared to a smooth-surface panelhaving no such microstructures.

It is therefore an object of this invention to provide an improvedwaveguide illumination system providing an efficient light extractionwith a minimum light loss and without using excessively deep (relativelyto the transversal size) microstructures in the waveguide's surface. Itis another object of this invention to eliminate or at leastsubstantially reduce the light spillage through the unwanted side of thewaveguide. It is yet another object of this invention to provide animproved waveguide illumination system which can be configured forenhanced light collimation and controlled directionality of the emittedbeam. It is yet another object of this invention to provide a waveguideillumination system capable of distributing light from a compact sourceover a large area and emitting the distributed light from said area inthe form of a collimated beam with a prescribed angular spread orpattern. It is yet another object of this invention to provide animproved waveguide illumination system which can effectively used as abacklight or a front light panel that will not substantially alter thelight paths and apparent image fidelity for the viewer. Other objectsand advantages of this invention will be apparent to those skilled inthe art from the following disclosure.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to waveguide illuminationsystems which may be employed to emit directional light beams oruniformly illuminate a designated area with a very low light loss. Moreparticularly, at least some embodiments of this invention are directedto planar light-emitting waveguides and at least some embodiments aredirected to cylindrical side-emitting waveguides and optical fibers.This invention is also directed to directional (collimating)illumination systems which employ light-emitting waveguides, such aslighting luminaires, backlights, front lights and the like.

The present invention solves a number of light distribution andillumination problems within a compact optical system which is nothindered by the limitations of conventional waveguides employing variouskinds of light extraction features used to decouple light from thewaveguide mode.

An advantage of the present system is to provide controlled lightextraction through a designated surface of the waveguide whileminimizing light loss and controlling the angular distribution of theextracted light. Light is extracted from the waveguide into anintermediate layer by means of incremental deflections from theprevailing propagation direction after which it is further redirectedout of the waveguide.

A two-stage light extraction process enables the directionality of theemitted light and minimizes light spillage into non-functionaldirections.

In at least one embodiment, the invention features a multi-layer opticalstructure having a waveguide layer, an intermediate buffer layer and alight extraction layer. Various implementation of the invention includea planar configuration of the waveguide and a cylindrical configurationof the waveguide.

The buffer layer has a lower refractive index than the waveguide layerand preserves the waveguiding function of the waveguide layer at leastfor a range of incidence angles. The waveguide includes light deflectingelements distributed along the intended path of light propagationconfigured to incrementally deflect light rays by relatively smallangles upon each interaction. According to an aspect of the presentinvention, the differential between the refractive indices at theopposing surfaces or sides of the waveguide and the smallness of thedeflection angles ensure light extraction into the buffer layer whilegenerally preventing light escape through the surface portion which isnot disposed in optical contact with buffer layer. The light extractionlayer further extracts and redirects light out of the illuminationsystem.

In at least one embodiment, the light deflecting elements includerelatively low-profile surface relief features deflecting light by meansof a total internal reflection (TIR). Each surface relief feature mayhave at least one facet which forms a relatively low dihedral angle witha surface plane. According to an aspect, the dihedral angle may besufficiently low to prevent premature light leakage from waveguidethrough the respective facet. According to another aspect, eachinteraction of light with the facet results in light deflection from itsoriginal propagation path by means of TIR and bends further away from aprevailing plane or axis of the waveguide. According to a furtheraspect, this process may continue until TIR is suppressed at least atone surface of the waveguide and light exits from the waveguide and maybe further redirected by the light extraction layer. In at least oneimplementation, the surface relief features include low-profileprismatic surface relief features. In at least one implementation, thesurface relief features include shallow surface undulations orcorrugations.

In at least one implementation, the dihedral angle of the facets variesacross the surface as a function of a distance from a light input areaof the waveguide. In at least one implementation, the waveguide includestwo symmetrically disposed segments each having a light input edge orend and each provided with an array of light-deflecting surface relieffeatures. In at least one implementation, the waveguide includes alinear array of surface relief features extending parallel to areference line. In at least one implementation, the waveguide includes atwo-dimensional array of discrete surface relief features. In at leastone implementation, the waveguide illumination system is configured fora generally unimpeded transversal light passage through its body. In atleast one implementation, the waveguide illumination system issubstantially transparent at least along a direction normal to itswaveguiding surface.

In at least one embodiment, the light deflecting elements include aninternal corrugated boundary between two optically transmissivematerials having different refractive indices. In at least oneembodiment, the light deflecting elements include light scatteringparticles distributed throughout the body of the waveguide andconfigured to continuously change the light propagation direction bymeans of forward scattering. In at least one embodiment, the lightdeflecting elements include an internal corrugated boundary between twooptically transmissive materials having different refractive indices.

In at least one embodiment, the waveguide illumination system of thisinvention includes at least one light source configured to input lightinto the waveguide. In at least one embodiment, the light source isoptically coupled to a light input edge or a light input end of thewaveguide. Various implementations of the light source include lightemitting diodes (LEDs), LED arrays, fluorescent lamps, incandescentlamps, cold-cathode or compact fluorescent lamps, halogen,mercury-vapor, sodium-vapor, metal halide, electroluminescent lamps orsources, lasers, etc. In at least one embodiment, the light source mayinclude various light-collimating or beam shaping elements. In at leastone implementation, the light extraction layer includes a light turningfilm or structure. In further implementations, the light turning film orstructure may include one or more microstructured surfaces, one or moreoptically transmissive layers, one or more inter-layer corrugatedboundaries or a reflective layer. In at least one implementation, thelight turning film or structure includes a plurality of reflective orrefractive facets inclined at an angle to the layer's surface. Thereflective or refractive surfaces may be formed by prismatic grooves,notches, undercuts or other type of surface modification. Alternativeimplementations of the light extraction layer include a screencomprising a scattering layer or image print.

In at least one embodiment, the waveguide illumination system of thisinvention is incorporated into a daylighting system. According to anaspect, the waveguide illumination system is configured and used as ahybrid luminaire combining natural and artificial illumination. Suchwaveguide illumination system has a layered panel structure whichtransmits sunlight delivered from a skylight in a transversal directionwith respect to the prevailing plane of the panel and distributes andemits light received from an array of LEDs coupled to an edge of thewaveguide. In at least one embodiment, the waveguide illumination systemof this invention is configured as an edge-lit front light for an imagescreen which provides high optical transparency and image fidelity whileefficiently illuminating the underlying image. In at least oneembodiment, the waveguide illumination system of this invention isconfigured as an edge-lit backlight. In at least one embodiment, thewaveguide illumination system of this invention is incorporated into alighting luminaire with improved beam directionality. According to anaspect, the edge-lit waveguide panel distributes and emits light from abroad-area surface of the panel in the form of a collimated beam.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 is a schematic perspective view of a conventional light emittingwaveguide employing surface microstructures for light extraction.

FIG. 2 is a schematic perspective view of a conventional light emittingwaveguide employing V-shaped grooves for light extraction.

FIG. 3 is a schematic perspective view of a conventional side-emittingoptical fiber employing cuts or notches formed in the fiber surface forlight extraction.

FIG. 4 is a schematic cross-sectional view of a conventionallight-emitting waveguide portion, showing the principles of lightextraction.

FIG. 5 is a schematic perspective view of a waveguide illuminationsystem, according to at least one embodiment of the present invention.

FIG. 6 is a schematic cross-sectional view and raytracing of a waveguideillumination system portion, according to at least one embodiment of thepresent invention.

FIG. 7 is a schematic cross-sectional view and raytracing of a waveguideillumination system portion, showing light reflection by a surfacerelief feature, according to at least one embodiment of the presentinvention.

FIG. 8 is a schematic cross-sectional view and raytracing of a waveguideillumination system portion, showing light interaction with an opposingsurface of a waveguide, according to at least one embodiment of thepresent invention.

FIG. 9 is a schematic cross-sectional view of and raytracing of awaveguide illumination system portion, showing the operation of a lightturning film attached to a buffer layer, according to at least oneembodiment of the present invention.

FIG. 10 is a schematic view of a waveguide illumination system portion,illustrating the principles of light collimation, according to at leastone embodiment of the present invention.

FIG. 11 is a schematic view of a waveguide illumination system, showinga buffer layer having a corrugated boundary with a light extractionlayer, according to at least one embodiment of the present invention.

FIG. 12 is a schematic cross-sectional view and raytracing of anillumination system portion, showing surface microstructure formed in alight extraction layer, according to at least one embodiment of thepresent invention.

FIG. 13 is a schematic cross-sectional view of and raytracing of awaveguide illumination system portion, showing a light scattering layerattached to a buffer layer, according to at least one embodiment of thepresent invention.

FIG. 14 is a schematic cross-sectional view of a waveguide illuminationsystem portion in a front light configuration, showing an exemplarylight path between a light scattering layer and a viewer, according toat least one embodiment of the present invention.

FIG. 15 is a schematic cross-sectional view of a waveguide illuminationsystem portion, showing a translucent light scattering layer, accordingto at least one embodiment of the present invention.

FIG. 16 is a schematic cross-sectional view of a waveguide illuminationsystem portion, showing a smooth corrugated surface of a waveguide,according to at least one embodiment of the present invention.

FIG. 17 is a schematic cross-sectional view of a waveguide illuminationsystem portion, showing a cladding layer attached to a corrugatedsurface of a waveguide, according to at least one embodiment of thepresent invention.

FIG. 18 is a schematic view of a waveguide illumination system having acylindrical configuration, according to at least one embodiment of thepresent invention.

FIG. 19 is a schematic view and raytracing of a waveguide illuminationsystem, showing multiple reflections from surface relief features,according to at least one embodiment of the present invention.

FIG. 20 is a schematic view and raytracing of a waveguide illuminationportion, showing a light collimator associated with a light input edgeof a waveguide, according to at least one embodiment of the presentinvention.

FIG. 21 is a schematic view of a waveguide illumination system includinga wedge-shaped waveguide and a light turning film, according to at leastone embodiment of the present invention.

FIG. 22 is a schematic view of a waveguide illumination system includinga wedge-shaped waveguide and a light scattering layer, according to atleast one embodiment of the present invention.

FIG. 23 is a schematic view of a waveguide illumination system, showinga light extraction layer having a mirrored surface, according to atleast one embodiment of the present invention.

FIG. 24 is a schematic view of a waveguide illumination system,illustrating a light collimation function of the system, according to atleast one embodiment of the present invention.

FIG. 25 is a schematic view of a waveguide illumination system used inconjunction with a daylighting device, according to at least oneembodiment of the present invention.

FIG. 26 is a schematic view of a waveguide illumination system portion,showing a buffer layer attached to a corrugated surface of a waveguide,according to at least one embodiment of the present invention.

FIG. 27A through FIG. 271 show various configurations of surface relieffeatures, according to at least some embodiments of the presentinvention.

FIG. 28 is a schematic elevation view of a waveguide illuminationsystem, showing a linear arrangement of parallel surface relief featuresformed in a waveguide surface, according to at least one embodiment ofthe present invention.

FIG. 29 is a schematic elevation view of a waveguide illuminationsystem, showing a two dimensional array of surface relief features,according to at least one embodiment of the present invention.

FIG. 30 is a schematic view of a waveguide illumination system portion,showing a buffer layer and a back-scattering light extraction layerattached to a waveguide surface comprising surface relief features,according to at least one embodiment of the present invention.

FIG. 31 is a schematic view of a waveguide illumination system portion,showing a light diffusing layer, according to at least one embodiment ofthe present invention.

FIG. 32 is a schematic view of a waveguide illumination system portion,showing light redirecting prismatic grooves within a light diffusinglayer, according to at least one embodiment of the present invention.

FIG. 33 is a schematic view of a waveguide illumination system portion,showing light redirecting slits within a light diffusing layer,according to at least one embodiment of the present invention.

FIG. 34 is a schematic view of a waveguide illumination system, showingsurface relief features having a variable slope, according to at leastone embodiment of the present invention.

FIG. 35 is a schematic view of a waveguide illumination system, showingtwo opposing arrays of surface relief features and two light sourcescoupled to opposing edges or ends of a waveguide, according to at leastone embodiment of the present invention.

FIG. 36 is a schematic exploded view of a front light implementation ofa waveguide illumination system, according to at least one embodiment ofthe present invention.

FIG. 37 is a schematic cross-sectional view and raytracing of awaveguide illumination system, showing light scattering features withina waveguide, according to at least one embodiment of the presentinvention.

FIG. 38 is a schematic cross-sectional view and raytracing of awaveguide illumination system, showing light scattering features withina waveguide and a buffer layer positioned between the waveguide and alight extraction layer, according to at least one embodiment of thepresent invention.

FIG. 39 is a schematic cross-sectional view and raytracing of awaveguide illumination system in a two-sided backlight implementation,according to at least one embodiment of the present invention.

FIG. 40 is a schematic cross-sectional view and raytracing of anillumination system portion illustrating the light extracting operationof scattering particles in conjunction with a low-index layer, accordingto at least one embodiment of the present invention.

FIG. 41 is a schematic cross-sectional view and raytracing of anillumination system portion illustrating the light extracting operationof forward-scattering particles in conjunction with a low-index layer,according to at least one embodiment of the present invention.

FIG. 42 is a schematic cross-sectional view and raytracing of awaveguide further illustrating a forward-scattering operation of lightscattering features, according to at least one embodiment of the presentinvention.

FIG. 43 is a schematic view of a scattering pattern characterizing anexemplary forward-scattering particle, according to at least someembodiments of the present invention.

FIG. 44 is a schematic view of a scattering pattern characterizinganother exemplary forward-scattering particle, according to at leastsome embodiments of the present invention.

FIG. 45 is a schematic view of an illumination system in anaxisymmetrical configuration, showing a light source in the center,according to at least one embodiment of the present invention.

FIG. 46 is a schematic view of an illumination system in an alternativeaxisymmetrical configuration, showing a plurality of light sources in acentral ring area, according to at least one embodiment of the presentinvention.

FIG. 47 is a schematic cross-sectional view and raytracing of anillumination system portion, showing a reflective layer adjacent to awaveguide and a right-angle turning film adjacent to the same waveguide,according to at least one embodiment of the present invention.

FIG. 48 is a schematic elevation view of a waveguide illuminationsystem, showing an annular arrangement of parallel surface relieffeatures formed in a waveguide surface, according to at least oneembodiment of the present invention.

FIG. 49 is a schematic cross-sectional view and raytracing of awaveguide portion, showing a corrugated boundary between twotransmissive materials having different refractive indices, according toat least one embodiment of the present invention.

FIG. 50 is a schematic perspective view of a directional lightingfixture employing a collimating illumination system, according to atleast one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring more specifically to the drawings, for illustrative purposesthe present invention is embodied in the apparatus generally shown inthe preceding figures. It will be appreciated that the apparatus mayvary as to configuration and as to details of the parts withoutdeparting from the basic concepts as disclosed herein. Furthermore,elements represented in one embodiment as taught herein are applicablewithout limitation to other embodiments taught herein, and incombination with those embodiments and what is known in the art.

A wide range of applications exist for the present invention in relationto the collection of electromagnetic radiant energy, such as light, in abroad spectrum or any suitable spectral bands or domains. Therefore, forthe sake of simplicity of expression, without limiting generality ofthis invention, the term “light” will be used herein although thegeneral terms “electromagnetic energy”, “electromagnetic radiation”,“radiant energy” or exemplary terms like “visible light”, “infraredlight”, or “ultraviolet light” would also be appropriate.

The present invention seeks to provide waveguide illumination systemscapable of progressively extracting at least a substantial portion oflight propagating in a waveguide and emitting the extracted light from awaveguide's side wall towards one or more predetermined directions in acontrolled manner and without substantial changing of the waveguidesurface smoothness and/or continuity, such as introducinghigh-aspect-ratio cuts, notches or grooves.

According to the present invention, there is provided an illuminationsystem employing a waveguide. The waveguide is configured to guide lighttoward a predetermined direction by means of a Total Internal Reflection(TIR) from its opposing walls having substantially smooth surfaces anddefining a waveguide core. The waveguide core should preferably be madefrom a material having good broadband optical clarity and transmission.When the illumination system is designed to operate in a specificspectral range, the material should be highly transmissive at least inthat spectral range. The waveguide core may be manufactured from glassor a suitable polymeric material including but not limited to opticalquality PMMA (acrylic), silicone, polycarbonate, PET (polyethyleneterephthalate), polystyrene, polyolefin, polyesters, APET, PETG, or PVC,as well as any optically clear resin which is obtainable bypolymerization and curing of various compositions. The waveguide may beformed by a single layer of the appropriate optically transmissivematerial or it may also include any number of additional layers madefrom the same or different materials having sufficient optical clarityfor light guiding purposes.

In one exemplary case of an edge-lit lighting waveguide panel, thewaveguide may be configured to receive light from a light source on oneedge and guide the light toward an opposing terminal end or edge. Inanother exemplary case of a planar waveguide having an axisymmetrical orfree-form configuration, light may me input through an opening in thecentral area of the waveguide and subsequently propagate radially fromthe input area towards the outer edge. In a further exemplary case of anoptical fiber or a cylindrical-configuration light pipe, the waveguidemay be configured to guide light from a first terminal end to anopposing second terminal end. Once light is input into the waveguide andits propagation angles permit for TIR to occur at waveguide's one ormore major surfaces, it becomes trapped within the core boundaries andcan propagate considerable distances until it is extracted, absorbed orit reaches the opposing edge, outer edge or terminal end of thewaveguide.

The present invention is generally directed to edge-lit planarwaveguides emitting from a broad-area surface and to end-lit,side-emitting cylindrical waveguides. Accordingly, when the waveguidehas a planar configuration with parallel walls, each of the opposingmajor surfaces of the waveguide as well as the body of waveguide may becharacterized by a plane which may be referred to as a prevailing planeof the respective element. Likewise, the illumination system based onthe waveguide may also have a well defined planar shape and may thusalso be characterized by a prevailing plane. It will be appreciatedthat, in case of the parallelism of the opposing broad-area waveguidesurfaces, the prevailing planes of the surfaces and the prevailing planeof the waveguide will be generally parallel to each other. Thus, when aparticular plane or a reference line makes an angle with respect to oneof those prevailing planes, it will also make the same angle with theother parallel prevailing planes. Particularly, when the term like“out-of-plane angle” is used to describe the angular relationship alight ray or reference geometry object such as plane or axis with one ofthe above reference planes, such term may also be generally applied tothe other parallel reference planes without limitations.

A cylindrical waveguide may be characterized by a prevailing axis, suchas a longitudinal axis of the cylindrical body forming the waveguide. Inthe context of the present inventions, as well as for the purpose ofillustrating the operation of the presently preferred embodiments, aside-emitting waveguide having a cylindrical configuration may also becharacterized by a prevailing plane. However, unlike the case of aplanar waveguide having well defined planes based on geometricaldimensions, the prevailing plane of a cylindrical waveguide may bedefined as a plane which is extending parallel to the prevailing axisand which is also generally separating the emitting and non-emittingsides of the waveguide.

The waveguide of the illumination system includes a light emittingregion associated with light deflecting elements distributed throughoutthe waveguide's body or throughout at least one of its major surfacealong the intended light propagation path. The function of the lightdeflecting elements is to cause a continuous change of the out-of-planeangle of a ray with the distance which the ray has propagated along thewaveguide. The light deflecting elements distributed along the opticalpath gradually deflect light from the original propagation direction inan incremental manner and eventually communicate such light a greaterangle with respect to a surface normal than the critical TIR angle thuscausing the light to exit from the waveguide at different locationsalong the extent of the waveguide. It is preferred that the deflectingelements are substantially non-absorbing so that the repetitiveinteraction of light rays with such elements does not cause perceptibleray attenuation along the propagation path. Suitable lossless ornear-lossless mechanism which may be employed for deflecting light raysinclude but are not limited to TIR and/or refraction at a boundarybetween two dielectric materials having different refractive indices.

The distance which a light ray may travel within the waveguide before itis extracted by overcoming TIR at the waveguide surface largely dependson the initial propagation angle. Generally, rays having greater initialout-of-plane (in case of a planar waveguide) or also out-of-axis (incase of a cylindrical waveguide) angle will travel shorter longitudinaldistances than rays initially propagating at more oblique angles withrespect to the waveguide's prevailing plane or axis, as they requireless interactions with the light deflecting elements to overcome TIR.However, it should be understood that the actual light path of each rayas well as its distance traveled within the waveguide may depend onother factors as well, particularly in view of the random character oflight propagation within the waveguide and random ray interactions withdeflecting elements.

In at least some embodiments of the present invention, the illuminationsystem includes a buffer layer or cladding layer attached to a majorwaveguide surface with a good optical contact. The buffer layer has alower refractive index than the waveguide but higher than that of theoutside medium. The buffer layer provides a differential in refractiveindex drop at the opposing major surfaces or sides of the waveguide andsuppresses TIR at least for some uttermost incidence angles compared tothe opposing boundary contacting with low-index medium (such as air).This causes the controlled leakage of the deflected out-of-plane raysprimarily through the designated major surface of the waveguidecontacting with the buffer layer rather than through both opposingsurfaces of the waveguide. Suitable materials for the buffer layer canadvantageously be selected for low-n fluoropolymers or resins, such as,for example, FEP, ETFE (both having a refractive index of 1.34-1.35),PFTE AF 1600 (n≈1.31), PFTE AF 2400 (n≈1.29), certain silicones, and thelike.

According to at least some embodiments, the light deflecting elementsinclude shallow surface relief features formed in the waveguide surface.The surface relief features are configured to slightly alter the angulardistribution of light upon each interaction and cause small portions oflight to overcome TIR and leak out of the waveguide's core into thebuffer layer while the main portion of can remain trapped within thewaveguide.

The surface relief features may be formed by shallow (low aspect ratio)recesses or depressions in the light guiding surface of the waveguide.Each surface relief feature may comprise a reflective face inclined tothe surface plane at a sufficiently low dihedral angle and configured toreflect light by means of TIR. The TIR surface of the reflective faceshould generally face the light input edge or end of the waveguide sothat it can be illuminated by a light source attached to that edge orend. The dihedral angle or slope of each reflective face with respect tothe waveguide surface should be substantially less than 45° and mayordinarily be less than 20° and, more preferably, less than 10°.According to at least one embodiment, the dihedral angle may take anangular value between 1° and 3°. According to at least one embodiment,the dihedral angle may be between 0.1° and 1°.

The upper practical limit for the dihedral angle of the reflective facemay be selected from various considerations depending on the intendedapplication of the illumination system. According to some embodimentsemploying the buffer layer, each reflective face should preferably beconfigured to reflect, by means of TIR, substantially the entire lightbeam impinging onto the reflective face back into the waveguide. Thismeans that the dihedral angle of the reflective face should be smallerthan a predetermined value defined by the differential between therefractive indices of the waveguide and the outside medium so as not tocause considerable light leakage through the reflective faces ofrespective surface relief features. According to some embodimentsconfigured for emitting collimated light from a major surface of thewaveguide, the dihedral angle of reflective faces may be selected basedon the desired degree of collimation.

The lower practical limit for the dihedral angle of the reflective facesmay be selected, for example, based on the desired rate of lightextraction that would ensure that most light injected into the waveguidecan be removed along the propagation path. Various other factors mayalso be considered, such as the spacing between individual lightextracting elements, the refractive index of the waveguide the adjacentlayers, and whether or not the uniformity of light emission from thewaveguide's surface is needed.

While each surface relief feature may have a very low aspect ratio(e.g., the ration between the feature's depth or height to its width orbase at the surface, it should be understood that the light deflectingportions of each reflective should still be essentially non-parallel tothe light guiding surface of the waveguide at least in a cross-sectionperpendicular to the prevailing direction of light propagation.Therefore, it will be appreciated that each of the reflective faces ofrespective light deflecting features will alter the light propagationangles upon each interaction with the guided light. Particularly, eachinteraction of light with the reflective face will broaden the angulardistribution of light propagating in the waveguide and cause at least aportion of reflected light to obtain greater out-of-plane angles andthus smaller incidence angles with respect to the TIR surface(s) of thewaveguide.

For the purpose of this discussion, the term “incidence angle” of alight ray in relation to a surface generally refers to an angle thatthis ray makes with respect to a normal to that surface. It will beappreciated by those skilled in the art of optics that, when referringto light or other waves passing through a boundary formed between twodifferent refractive media, such as air and glass, for example, theratio of the sines of the angles of incidence and of refraction is aconstant that depends on the ratio of refractive indices of the media(the Snell's law of refraction). The following relationship can describelight bending property of an interface between two refractive media:n_(I) sin ϕ_(I)=n_(R) sin ϕ_(R), where n_(I) is the refractive index ofthe material where the light is incident from, n_(R) is the refractiveindex of the material where the light refract to, and ϕ_(I) and ϕ_(R)are the angle of incidence and the angle of refraction, respectively. Itwill be further appreciated that such optical interface can also becharacterized by the angle of a Total Internal Reflection (TIR) which isthe value of ϕ_(I) for which ϕ_(R) equals 90°. Accordingly, for asurface characterized by a stepped drop in refractive index along theray propagation path, the incidence angle may be less than, equal to, orgreater than the TIR angle at the given surface.

A TIR angle ϕ_(TIR) can be found from the following expression:ϕ_(TIR)=arcsin(n_(R)/n_(I)·sin 90°)=arcsin(n_(R)/n_(I)). In an exemplarycase of the interface between acrylic with the reflective index n_(I) ofabout 1.49 and air with n_(R) of about 1, ϕ_(TIR) is approximately equalto 42°.

Since each reflective face of the surface relief features broadens theangular distribution of light propagating in the waveguide, at leastsome uttermost out-of-plane rays may obtain incidence angles withrespect to the boundary between the waveguide core and the buffer layerwhich are less than the TIR angles for the respective boundary.Therefore, these rays will refract into the buffer layer and thus exitfrom the waveguide core.

It will be understood that, due to the nature of incremental deflectionof light rays by relatively small angles using the light deflectingfeatures, the rays escaping from the waveguide due to less-than-TIRincidence angles will generally have a relatively narrow angular spread.Additionally, the rays emerging from the major surface of the waveguideand refracting into a smaller-index medium at near-TIR incidence angleswill have relatively low angles with respect to the waveguide surfaceand relatively high refraction angles. The angular spread of theemerging rays can be controlled by the appropriate configuration of thelight deflecting means and their distribution density along theprevailing light path in the waveguide. Particularly, by way of exampleand not limitation, the dihedral angle of reflective faces may beadvantageously selected to result in the refracted rays emerging atgrazing angles with respect to the waveguide surface (corresponding torefraction angles close to 90°) and having a fairly narrow angularspread. Accordingly, the subsequent surface relief featured distributedalong the waveguide will respectively deplete the remaining light fromthe waveguide and eventually extract it at grazing angles with respectto the surface plane. As the maximum deviation angle from the plane ofthe waveguide can be limited to a sufficiently low value, the resultinglight beam may have a high degree of collimation while propagatingnearly parallel to the longitudinal axis of the waveguide.

The shallow surface relief features may be formed by a variety ofsuitable means and may comprise any surface irregularities, undulationsor corrugations that slightly alter the reflection properties of thesurface and cause light reflection at different angles compared to aperfectly flat or straight surface. When the surface relief featureshave a linear or cylindrical geometry, the longitudinal axis of eachsuch feature should generally extend perpendicular to the prevailingdirection of light propagation on the waveguide.

The surface relief features may be fabricated together with thewaveguide where the waveguide core may be cast or molded using anegative replica of the surface relief features. Alternatively, thewaveguide may be fabricated first and the surface relief features may beformed in it by any suitable method for structural surface modification.Suitable methods may include laser ablation, chemical etching,embossing, grinding, polishing, molding, extrusion, material expansionor contraction, bending, etc. The surface relief features may also beformed in an external layer of optically transmissive material which maybe then attached to a major surface of the main waveguide's body with agood optical contact. Suitable surface relief features may also beformed by corrugations or bends of the waveguide as a whole, as well asby any other means causing portions of the surface to reflect light at agreater out-of-plane angle compared to an ideally-flat, smooth surface.

According to at least some embodiments of the present invention, thelight deflecting elements are formed by light scattering featuresdistributed throughout the body of the waveguide at least in its lightemitting region. Particularly, the light scattering features can beformed by small dielectric particles or by any other form of opticalirregularities in the otherwise homogenous body of the waveguide. By wayof example, the light scattering features may be formed by very smallproportions of imbedded, finely divided, spherical or asphericalparticles made from a transparent plastic or glass material whichrefractive index differs from that of the waveguide by a predeterminedamount.

The light scattering particles can be homogenously distributed in thevolume of the waveguide's body. Each particle can have a plain structureor a core-shell structure such as, for example that of known core-shellparticles obtainable by emulsion polymerization. The light scatteringfeatures are configured to deflect light propagating in the waveguidefrom the original propagation path by means of forward scattering andcommunicate greater angles to said light with respect to a normal to thewaveguide surface. Each light scattering feature should be designed tolimit the scattering angle within a relatively narrow cone. Thus, aseries of light scattering features on the optical path may thus providefunction of incremental light deflection somewhat similar to thefunction of shallow surface microstructures explained above. Single ormultiple interactions of light propagating by means of TIR in thewaveguide with such light scattering features will result in theextraction of relatively small portions of light from the waveguide coreat different locations across the waveguide's surface. The structure andoptical properties of the light scattering features should be preferablyselected to result in light out coupling from the waveguide's core atoblique angles with respect to the surface. When the buffer layer isemployed, light should primarily leak out of the waveguide into thislayer rather than exit from the opposing surface of the waveguide.

According to at least some embodiments of the present invention, theillumination system may be further provided with a light extraction orlight distribution layer adjacent to a major surface of the waveguide.The main function of the light extraction or distribution layer is tofurther direct or distribute the light extracted by the light deflectingelements. When the buffer or cladding layer is employed, the lightextraction or distribution layer may be externally attached to thebuffer layer with a good optical contact or made an integral part of thebuffer layer.

By way of example and not limitation, the light extraction layer maycomprise a light turning film or structure. The light turning film mayredirect the collimated light beam emerging from the waveguide towards aperpendicular of the waveguide surface thus providing a useful source ofdirectional light emitted along the entire length of the light emittingregion of the waveguide. At least some types of light turning or lightredirecting films that may be incorporated into the light extractionlayer of the present invention are disclosed in co-pending, co-ownedapplication Ser. No. 13/662,311 which is incorporated by reference inits entirety herein.

In a non-limiting example, the light turning film may be of a refractivetype configured for turning the collimated light emerging from thewaveguide by approximately 90° away from the waveguide surface. In analternative non-limiting example, the light turning film may be of areflective type and may direct light through the waveguide and alsoperpendicular to its surface.

According to at least some embodiments, the light extraction layer mayinclude a screen comprising a scattering layer or image print. The imageprint may be printed or otherwise deposited directly on the externalsurface of the buffer layer. Alternatively, the image print may beprovided on a transparent substrate which can be attached to the bufferlayer. The screen may reflect and/or scatter light propagating in thebuffer layer toward an observer located at a distance from thewaveguide. According to different variations of this invention, thelight scattering screen may be configured to provide forward scattering,back scattering or any combination of the two. Particularly, the screenmay be made opaque with back-scattering properties only, in which casethe illumination system may be used as a front light. Alternatively, thescreen may be made at least partially transmissive with the forwardscattering function, in which case the illumination system may be usedas a backlight. Furthermore, the transmissive properties of the screenmay be adjusted so that the screen can be lit by the light emerging fromthe waveguide and made visible from both sides, thus forming a two-sidedillumination system. Useful examples of the light extraction ordistribution layer may also include light scattering surfaces or films,phosphorescent or fluorescent films, light filtering films or layers,diffusers, and the like.

When the light extraction layer includes a viewable screen and thewaveguide is positioned between the screen and the viewer in afront-light configuration, the surface relief features may be madesubstantially smooth and shallow so that they will not substantiallyalter the smoothness and continuity of the waveguide surface and willnot notably bend the path of light propagating at near-normal angleswith respect to the waveguide surface.

Various layers employed in the waveguide illumination system, may beattached to each other or to the respective surfaces using any suitablemethod providing a good optical contact. For example, any two layers maybe simply laminated onto each other with no air gaps. Alternatively, anyintermediate layers may be used such as optical adhesives or two-sidedtransparent adhesive films to promote optical and physical contact. Therespective layers of the illumination system may also be attached toeach other by chemical bonding, heat bonding, ultrasonic bonding,welding, etc.

According to at least some embodiments of this invention, the waveguideillumination system can be made optically transmissive in a transversaldirection and configured for a generally unimpeded transversal lightpassage through its body. In addition to that, the system may beconfigured to emit collimated light from a selected broad-area surfaceor its portion and limit light emission from the opposing surface. Inother words, the waveguide illumination system may be configured toprovide directional illumination from one side with the prescribeddegree of collimation of the emitted beam while precluding or at leastsubstantially reducing light escape from the opposing side andpreserving the transversal optical transmissivity. In contrast to theprior art illumination systems, the waveguide illumination system ofthis invention may be configured to not require using any opaque layersto prevent light decoupling from an unwanted side or surface.

According to at least some embodiments of this invention, the waveguidecan be made both highly transmissive and transparent along the lightpath perpendicular to the waveguide's prevailing plane or axis.Particularly, the waveguide can be configured to have a very high visualtransparency at least in the direction along a normal to its broadsurface. In other words, in addition to having high lighttransmissivity, the waveguide of this invention may have the property oftransmitting light without appreciable scattering along a normal viewingdirection so that bodies or images lying beyond can be seen clearly.While the light deflecting elements of this invention are used todeviate light from the waveguiding light paths by means of multipleincremental deflections, they can still be configured to not appreciablyalter the propagation angles of light propagating perpendicular to theprevailing plane or axis of the waveguide. This is generally in contrastto the prior art illumination systems employing waveguides with othertypes of light extraction microstructures or scattering features whichcause deterioration of either one or both the transmissivity ortransparency.

In operation, the light deflecting elements alter the propagation anglesof light with respect to the prevailing plane or longitudinal axis ofthe waveguide by means of continuous incremental deflections along thepropagation path. Each deflection alters the propagation angle by arelatively small amount which allows most rays to propagate aconsiderable distance in a waveguide. Multiple interactions of lightrays with the light deflecting elements continues until at least theuttermost out-of-plane rays obtain less-than-TIR incidence angles withrespect to the waveguide surface and exit from the waveguide core atrelatively low angles with respect to the prevailing plane of thewaveguide. When the buffer layer is employed, it creates a differentialin the refractive index drop at the opposing surfaces or longitudinalsides of the waveguide. In turn, it creates a preference for light raysto exit from the waveguide through the side or surface to which thebuffer layer is attached. The light extraction layer intercepts thelight emerging from the waveguide and directs it further at higherangles with respect to the prevailing plane of the waveguide, thusfinally extracting light from the illumination system and directing ittowards one or more predetermined directions.

The present invention will now be described by way of example withreference to the accompanying drawings.

FIG. 5 depicts an embodiment of a waveguide illumination system 2 inaccordance with the invention. System 2 includes a waveguide 4exemplified by a rectangular, planar slab waveguide having generallysmooth-surface walls configured to conduct light by means of TIR.Waveguide 4 is configured to have a first major broad-area surface 10and an opposing major broad-area surface 12 extending generally parallelto surface 10. Waveguide 4 also has four edges, one of the edged beingdesignated as a light input edge and the opposing edge being designatedas a terminal edge. As waveguide 4 may be associated with additionallayers attached to either of its major surfaces or edges, the mainwaveguide body defined by its major surfaces 10 and 20 and by the fouredges is also hereinafter referred to as a waveguide core.

A light source 400 is provided on the light input edge so that waveguide4 can guide light from the light input edge towards the opposingterminal edge by mean of TIR which involves bouncing light from at leastsurfaces 10 and 12. The prevailing direction of light propagation inwaveguide 4 defines a longitudinal axis 200 of the waveguide. Thewaveguide edges extending parallel to the longitudinal axis (thelongitudinal edges) may also be made smooth and polished in order to beable to guide light by means of TIR. Waveguide 4 preferably has arefractive index sufficiently greater than the refractive index of theoutside medium to provide for the TIR light guiding properties in apredetermined acceptance angle.

Light source 400 may include any suitable single or multiple lightsources of any known type. According to one embodiment, light source 400may include one or more light emitting diodes (LEDs). Multiple LEDs maybe arranged in a linear strip or a two-dimensional array. Whenhigh-brightness LEDs are employed, as may be the case, for example, whensystem 2 is employed in an overhead lighting panel, other type ofwide-area luminaire, the LEDs may also be provided with a heat sink toremove excess heat generated by the LED chips. A power supply (driver)may also be provided which electrical characteristics may be matchedwith those of the LED light source 400. Furthermore, a suitable supportframe or housing may also be provided to hold waveguide 4 and source 400with the associated components together and/or encase all or at leastsome parts of system 2.

It is noted that light-emitting devices suitable for light source 400are not limited to LEDs and may also include fluorescent lamps,incandescent lamps, cold-cathode or compact fluorescent lamps, halogen,mercury-vapor, sodium-vapor, metal halide, electroluminescent lamps orsources, lasers, etc. Each light source may have any suitable shape,including compact two-dimensional or elongated one-dimensional shapes.

Light source 400 may have integrated optics such as collimating orlight-redistributing lenses, mirrors, lens arrays, mirror arrays, lightdiffusers, waveguides, optical fibers and the like. When light source400 includes a series of compact light sources, such as LEDs, each LEDmay be provided with individual collimating optics or, alternatively, asingle collimating optical element may be supplied to inject light fromall of the LEDs into the edge of waveguide 4. Numerous applications ofsystem 2 exist where waveguide 4 may have a planar slab configurationand where light source 400 may be associated with a strip of high-powerLEDs optically coupled to the light input edge of the waveguide.

Surface 10 of waveguide 4 has at least one light emitting regioncomprising light deflecting elements exemplified by surface relieffeatures 8 formed in surface 10. More particularly, surface relieffeatures 8 are represented by repetitive shallow depressions formed in astepped arrangement in surface 10. The depressions can be characterizedby alternating peaks and valleys connected by smooth, sloped surfaceportions. Surface relief features 8 preferably have a linear geometrywith a common longitudinal axis extending generally perpendicular to thelongitudinal axis 200 of waveguide 4. The shallow depressions formingsurface relief features 8 slightly alter the structure of surface 10 yetallowing the surface to remain generally smooth and planar. Sincesurface 10 is optically transmissive, the shallow surface relieffeatures 8 formed in this surface generally preserve both longitudinaland transversal transmissivity of waveguide 4. Furthermore, surface 10can be characterized by a first TIR angle ϕ_(TIR1) which can be foundfrom the expression: ϕ_(TIR1)=arcsin(n₀/n₁), where n₀ is the refractiveindex of the outside medium and n₁ is the refractive index of the coreof waveguide 4. Ordinarily, the outside medium can be air with n₀≈1, inwhich case ϕ_(TIRI)≈arcsin(1/n₁).

Waveguide 4 further comprises a buffer layer 6 disposed in a goodoptical and physical contact with surface 12 at least in the lightemitting region. Buffer layer 6 has a refractive index lower than therefractive index of waveguide 4 and may also have a function of acladding layer for the core of waveguide 4. Accordingly, the interfacebetween the core of waveguide 4 and buffer layer 6 may be characterizedby a second TIR angle ϕ_(TIR2) which can be found from the followingexpression: ϕ_(TIR2)=arcsin (n₂/n₁), where n₂ is the refractive index ofbuffer layer 6. The system may also be characterized by a critical TIRangle ϕ_(TIRC) which is the greater of the first and second TIR anglesϕ_(TIR1) and ϕ_(TIR2), respectively. When features 8 are sufficientlyshallow and make very low dihedral angles with the plane of surface 10,the critical TIR angle ϕ_(TIRC) generally defines a minimum incidenceangle that the light propagating in waveguide 4 must have with respect anormal to the longitudinal walls of the waveguide's core in order toremain being guided through the core by means of TIR.

Referring further to the embodiment illustrated in FIG. 5 , n₀<n₂<n₁and, consequently, ϕ_(TIR2)>ϕ_(TIR1) and ϕ_(TIRC)=ϕ_(TIR2). It will beappreciated, as long as the incidence angles of light rays onto surfaces10 and 12 remain above ϕ_(TIRC), light can propagate within the core ina waveguide mode.

As it will be described in detail below, surface relief features 8 areconfigured to redirect light within waveguide 4 so that predefinedportions of light are eventually communicated incidence angles which areless than ϕ_(TIR2) but still substantially greater than ϕ_(TIR1).Maintaining the incidence angle generally above ϕ_(TIR1) ensures thatsurface 10 and its portions formed by features 8 will continue to bereflective by means of TIR. On the other hand, making the incidenceangle less than ϕ_(TIR2) for a small portion of light causes theextraction of the respective portion from the waveguide core into layer6. In other words, a major function of surface relief features 8 is tocause a controlled leakage of light from the core of waveguide 4 intobuffer layer 6 through surface 12 along the propagation path, yetpreventing light escaping through surface 10.

While the light-bending properties of surface relief features 8 of theabove-illustrated embodiment are selected to be insufficient to extractlight from system 2 without additional means, they nevertheless play animportant role in system 2 operation. Surface relief features 8 actcooperatively with layer 6 to preliminary extract light from thewaveguide core into the buffer layer so that light can be furtherdirected and distributed with improved efficiency using additional lightextraction features. The refractive index of buffer layer 6 and theproperties of surface relief features 8 can be configured to recoverlight from the core of waveguide 4 through surface 12 at any desirablerate along the propagation path and without causing light loss throughsurface 10. Accordingly, system 2 further includes a light extractionlayer 20 exemplified by a light turning film disposed in contact with anexternal surface 14 of buffer layer 6.

Surface 14 is opposing the broad surface of buffer layer 6 that iscontacting waveguide 4. The light turning film comprises two transparentlayers having different refractive indices and separated by a corrugatedboundary between the layers. The film is configured to extract lightfrom buffer layer 6 and redirect it toward a designated direction whichmay be advantageously selected to be normal to the plane of waveguide 4.

An optional specularly reflective or diffusively reflective layer (notshown) may be provided and positioned adjacent to surface 10 ofwaveguide 4 to reflect any stray light that may escape through surface10 outside of the waveguide. The stray light may include, for example,rays that are scattered by impurities in the materials of waveguide 4,layer 6 or layer 20, as well as by imperfections of surface relieffeatures 8 or the corrugated boundary within the light turning film.

FIG. 6 illustrates further details and operation of system 2 depicted inFIG. 5 . Each surface relief feature 8 comprises a first face 16generally facing light source 400 and a second face 18 generally turnedaway from source 400. Both faces 16 and 18 are ordinarily planar andeach form non-zero dihedral angles with the prevailing plane of surface10. These dihedral angles may also be referred to as slope angles of therespective faces to surface 10. The slope of each face 16 is preferablyselected to be fairly low and substantially lower than the slope of therespective face 18 so that faces 16 have considerably greater surfacearea than faces 18. Accordingly, this translates into a considerablylonger cross-sectional profile of face 16 compared to the profile offace 18. The slope of each face 18 may be advantageously selected toensure that the face is completely shaded from source 400 by therespective face 16. Particularly, the angle which face 18 makes with anormal to the prevailing plane of surface 10 is preferably smaller thanϕ_(TIRC). This can ensure that light propagating in the waveguide modewithin the core of waveguide 4 will never strike any of faces 18 andwill interact only with faces 16. Suitable angles for faces 18 mayinclude normal or near-normal angle with respect to surface 10.

In operation, source 400 illuminates the light input edge of waveguide 4with a divergent beam which causes at least a substantial part of thebeam to enter the waveguide core at angles permitting for TIR. Waveguide4 further guides light toward the opposing terminal edge by bouncingsaid light from the opposing parallel surfaces 10 and 12. Since surfacerelief features 8 are sufficiently shallow and the slopes of faces 16are low, the change in the propagation angle with respect to axis 200 isalso low. Therefore, most light reflected by each face 16 continues itspropagation in the waveguide by means of TIR while incrementallyobtaining a slightly broader angular distribution with respect to axis200 at each interaction with surface 10 in the light emitting region.

It will be appreciated that, provided that there is a sufficient opticalpath along the waveguide's longitudinal axis, the incremental deviationof a light ray from axis 200 will eventually result in said ray reachingsurface 12 at an incidence angle which is less than ϕ_(TIRC). This, inturn, will ultimately cause ray extraction into buffer layer 6.Obviously, light rays having relatively small out-of-plane angles willgenerally undergo morel bounces from faces 16 before reaching sub-TIRangles and before being extracted from the core of waveguide 4 than rayshaving larger out-of-plane angles. For example, a ray 72 strikes face 16of one of the surface relief features 8 and is losslessly reflected byTIR back into waveguide 4. While the reflection from face 16 increasesthe out-of-plane angle of ray 72, the incidence angle with respect tosurface 12 still remains greater than the TIR angle ϕ_(TIR2) at theinterface between the waveguide 4 and buffer layer 6. Therefore, ray 72undergoes TIR from surface 12 and continues to be guided by means ofTIR. Accordingly, longer optical path and additional interactions withsurface relief features 8 will be needed to extract ray 72 into bufferlayer 6.

In contrast, light propagating in waveguide 4 at angles close to thecritical TIR angle ϕ_(TIRC) can be extracted into buffer layer 6 nearthe light input edge of the waveguide, as illustrated by the path of aray 74. Ray 74 is emanated by the same light source 400 but has agreater initial out-of-plane angle than ray 72. Ray 74 strikes face 16of an individual surface relief feature 8 at an incidence angle which isgreater than ϕ_(TIR1). Therefore, ray 74 is reflected from face 16 bymeans of TIR and is directed toward opposing surface 12 at a greaterout-of-plane angle than before striking feature 8. When the slope offace 16 is sufficient to result in a less than ϕ_(TIR2) incidence angleof ray 74 onto surface 12, no TIR will occur at surface 12 and ray 74will refract into buffer layer 6.

Ray 74 further propagates in buffer layer 6 towards opposing surface 14where it enters the light turning film of light extraction layer 20.Layer 20 is preferably configured to have a good optical contact withbuffer layer 6. Although it may be simply laminated onto surface 14 withno air bubbles, an adhesion promoting layer may also be used, such as alayer of optical adhesive of double-sided adhesive tape or film, forexample. The refractive index of the inner layer of the light turningfilm contacting surface 14 should preferably be not less than therefractive index of buffer layer 6. Likewise, the refractive index ofthe adhesion promoting layer, is any, should also be no less than therefractive index of layer 6. According to some embodiments, the aboverefractive indices may be matched to each other in order tosubstantially reduce or eliminate the Fresnel reflections.

The outer layer of the light turning film may have a refractive indexgreater than its inner layer. The corrugated boundary between the innerand the outer layer acts as a prismatic array and redirects light at adifferent angle with respect to the surface or its normal. Theredirection mechanism may involve refraction and/or TIR. Accordingly,the internal boundary corrugations of the light redirecting film may beconfigured to intercept rays propagating at near-grazing angles inbuffer layer 6 and redirect them towards a normal to the plane ofwaveguide 4, as illustrated in FIG. 6 referring to ray 74.

FIG. 7 depicts, in a cross-section parallel to the light propagationpath, a portion of waveguide 4 including an individual prismaticdepression in surface 10 and illustrates the light redirecting operationof surface relief feature 8 formed by said surface depression.

For the purpose of clarity and explaining the principles of lightredirection by surface 10, the individual surface relief feature 8 isshown surrounded by flat portions of surface 10 which are parallel tolongitudinal axis 200. However, it should be understood that system 2may have any number of surface relief features 8 which can be spacedapart, contacting each other, overlapping, or otherwise distributed withany prescribed density along the intended propagation path.

Surface relief features 8 may have a constant pitch or spacing.Alternatively, the spacing between adjacent features 8 can be madevariable along the propagation path. Particularly, it may beadvantageous to provide some initial spacing between surface relieffeatures 8 near the light input edge and gradually increase the densityof the features as the distance from the light input edge increases.This may help improve the uniformity of light emission from surface 12as the increasing density of surface relief features 8 will compensatethe depletion of light by the preceding features 8.

Referring further to FIG. 7 , light propagates in waveguide 4 left toright within an angular cone having an angular aperture of ±β, where βis an out-of-plane propagation angle counted from longitudinal axis 200.An uttermost ray 174 having the propagation angle of −β is shownstriking face 16 of surface relief feature 8 at a point 90.

Face 16 is inclined at a dihedral angle α, hereinafter also referred toas a slope angle α, with respect to the prevailing plane of surface 10.Angle α is selected to be sufficiently low in order to preserve TIR atface 16 and to not cause decoupling of ray 174 through surface 10. 16and result in TIR back to the core of waveguide 4. In will beappreciated that when light propagates in a waveguide mode, an anglecomplementary to angle β should generally exceed ϕ_(TIRC). Consideringthat, when low-n buffer layer 6 is employed (not shown in FIG. 7 ),ϕ_(TIRC)=ϕ_(TIR2) and ϕ_(TIR2)>ϕ_(TIR1), the acceptable range of angle αmay be generally defined by the following relationship:0<α<ϕ_(TIR2)−ϕ_(TIR1).

Accordingly, the uttermost ray 174 having the propagation angle −β uponentering point 90 reflects from face 16 by means of TIR and obtains anew propagation angle of β+2α, as a matter of optics. Therefore, theindividual surface relief feature 8 causes widening the lightpropagation cone in the core waveguide 4 by angle 2α and also causestemporary angular asymmetry of the cone by the same angle.

FIG. 8 depicts an upper portion of waveguide 4 and illustrates theinteraction of light with opposing surface 12 of waveguide 4 after itspassing surface relief feature 8 of FIG. 7 . Due to the smallness ofslope a and the resulting smallness of the deflection angle caused bysurface relief feature 8, a substantial part of light continues topropagate in the core of waveguide 4 by means of TIR. Particularly, allrays having the incidence angles with respect to surface 12 less than βare reflected from surface 12 by means of TIR. In other words, all lightremaining in the angular range ±β, with respect to longitudinal axis200, remains also confined in a waveguide mode.

However, the uttermost rays from the broadened angular propagation conemay now have incidence angles which no longer exceed second TIR angleϕ_(TIR2). As further illustrated in FIG. 8 , those rays may refract intobuffer layer 6, generally at relatively low acute angles γ with respectto surface 12.

In an illustrative example where 90°−β≈ϕ_(TIR2), substantially all ofthe light having the propagation angles greater than β will exit fromthe core of waveguide 4 into buffer layer 6. Thus, it will beappreciated that, after interacting with surface 12, the light beampropagating within waveguide 4 will shed a relatively narrow cone of 2αinto buffer layer 6 and again obtain the prior ±β angular range. Inother words, the optical interface formed by surface 12 and separatingwaveguide 4 from smaller-refractive-index buffer layer 6 is “shavingoff” rays having propagation angles in excess of the critical anglesthat still permit TIR at surface 12. The escaping cone 2α represents afixed portion of the angular distribution of light in waveguide 4. Thus,the slope α of face 16 determines the amount of light extracted by eachfeature 8 into buffer layer 6 along the optical path. Accordingly, therate of light extraction can be accurately controlled by varying theslope of TIR faces in the light extraction area for a given waveguide 4geometry and relative refractive indices of the waveguide core andbuffer layer 6.

It should be noted that a certain portion of light may undergoreflection from surface 12 even when the incidence angle onto saidsurface is less than ϕ_(TIR2), owing to the so-called Fresnel reflectionfrom the optical interface between two layers having differentrefractive indices. Light reflected from surface 12 by means of Fresnelreflection will thus return back into waveguide 4 and can be recycled.

When waveguide 4 has planar slab geometry, the shallow depressions of insurface 10 that form surface relief features 8 may be made by sheetcasting or extrusion from a suitable transparent polymer, such asacrylic, polystyrene or polycarbonate, for example. Alternatively,surface relief features 8 may be formed in a flat sheet of glass orpolymer by any suitable methods for micro-replication or materialremoval. For example, grinding, milling or fly-cutting may be used withsubsequent polishing of surface 10. In a more specific example, surfacerelief features 8 may be formed using a sharp diamond-tipped bit orcutter in which case the sufficient surface finish may be obtainedwithout the need of subsequent polishing. In an exemplaryimplementation, the diamond cutting tool having the appropriate shapeand slope of the cutting surface can be dragged across the surface ofwaveguide 4 leaving a shallow groove. In an alternative exemplaryimplementation, the diamond cutting tool can be used in a “fly-cutting”mode as it can be spun in a spindle at a high speed (preferably atspeeds of 20000 to 100000 RPM), plunged to the appropriate depth intothe surface of waveguide 4 and moved across the waveguide's surface. Therotation axis the tool should be preferably inclined at an angle withrespect to a normal to the prevailing plane of waveguide 4 correspondingto the desired slope angle of the face 16 to be formed. Features 8 maybe formed directly in the surface of waveguide 4 or they can also beformed in a separate optically transparent film or plate which can beattached to surface 10.

The structure and operation of light extraction layer 20 including alight turning film is illustrated in FIG. 9 by way of example. The lightturning film of layer 20 comprises a first layer 104 and a second layer106 disposed in contact with each other and having a corrugated boundarybetween the two layers. Corrugations 108 forming the boundary have alinear triangular configuration in a cross-section with peaks andvalleys extending perpendicular to the plane of drawing. Corrugations108 also define a plurality of alternating interface facets makingpredetermined angles with the prevailing plane of system 2 so that thecorrugated boundary between layers 104 and 106 comprises a plurality offacets 124 alternating with facets 126. Facets 124 are characterized bya first dihedral angle 162 with respect to the prevailing plane ofsystem 2 and facets 126 are characterized by a second dihedral angle 164with respect to that plane. Planes parallel to the prevailing plane ofsystem 2 are indicated by reference lines 142 and 148 in FIG. 9 . Thelight turning film is configured so that a refractive index n₃ of layer106 contacting buffer layer 6 is smaller than a refractive index n₄ oflayer 104 facing outwardly from layer 6.

The light turning film of layer 20 is configured to accept lightpropagating at low angles along surface 14 in buffer layer 6 andredirect said light at a greater angle with respect to the surface so asto result in light decoupling through surface 110. Referring further toFIG. 9 , an angle 160 represents the low angle that incident ray 74makes with surface 14 when it enters layer 20. In the illustratedembodiment, the light turning film may be configured to accept lightemerging from layer 6 at angles between 0° and 20° from surface 14(corresponding to 90° and 70° incidence angles with respect to a surfacenormal, respectively) and communicate said light a smaller angle withrespect to the surface normal so that TIR can be overcome at surface110. When system 2 is configured for light collimation, angle 160 shouldpreferably be within a predefined narrow range of angles from theprevailing plane of waveguide 4, said range being primarily defined bythe desired angular cone of the collimated light. While angle 160 mayrepresent very sharp, near-grazing angles with respect to surface 14, itshould also be understood that this angle may take any other suitableangular values provided that system 2 has the same basic operation.

Facets 124 are configured to have a generally smaller dihedral anglewith respect to the prevailing plane of system 2 than facets 126.Furthermore, the dihedral angle 162 of facets 124 is preferably selectedto be less than an angle which is complementary to angle 160 in order toprovide refraction towards a normal to that plane.

Dihedral angle 164 of each facet 126 is preferably made greater than amaximum designed value of the angle that light can make with surface 14in layer 104 after refracting at facet 124. At the same time, dihedralangle 164 of each facet 126 should preferably be selected so that thefacet 126 can intercept light refracted by a preceding adjacent facet124 and reflect it by means of TIR. Referring yet further to FIG. 9 ,light ray 74 deflected by the respective surface relief feature 8 (notshown) and receiving a sub-TIR angle with respect to surface 12 isextracted from the core of waveguide 4 (not shown) and enters bufferlayer 6. Ray 74 further crosses surface 14 and enters layer 106 of layer20. The refractive index n₂ of layer 6 can be matched to the refractiveindex n₃ of layer 106 so that ray 74 can make about the same refractionangle as the incidence angle with respect to a surface normal 140.

Ray 74 entering layer 106 at a sharp angle with respect to surface 14propagates in layer 106 until it strikes facet 124 of the corrugatedboundary with layer 104. Depending on angle 162, ray 74 may slightlybend toward a normal 144 by means or refraction at the interface betweenthe lower refractive index layer 106 and the higher refractive index oflayer 104, after which it may strike the next adjacent facet 126. Theslope of facet 126 defined by dihedral angle 164 is selected to resultin TIR at the interface between high-index layer 104 and low-index layer106. Upon TIR, facet 126 communicates an additional bend angle to ray74, this additional bend angle being twice the angle between ray 74 andfacet 126. As a result, ray 74 may exit from layer 20 nearlyperpendicular to surface 110. It should be understood that light turningfilm may also be configured to result in the emergence angles other thannormal. However, it should also be understood that, ordinarily, ray 74will be communicated an exit angle with respect to waveguide's 4prevailing plane which is substantially greater than angle 160.According to at least some embodiments of this inventions, when system 2is used for light distribution and improved collimation, it may bepreferred that the slopes of surface relief features 8 and otherparameters of waveguide and the respective outer layers are selected sothat the light beam emitted from a major broad area surface of system 2has a divergence which is at least less than the initial divergence oflight emitted by source 400.

FIG. 10 illustrates yet further details and operational aspects ofembodiments of system 2 shown in the preceding drawing figures anddepicts an exemplary case when system 2 is configured for emittingcollimated light from its broad-area surface. A fan of rays 802exemplifies the angular distribution of light initially propagating inwaveguide 4 before interacting with surface relief features 8. It willbe appreciated that the maximum half-angle of fan of rays 802 is definedby the acceptance angle of the core of waveguide 4. The acceptance angleis primarily defined by the refractive indices of waveguide 4 and bufferlayer 6.

As light passes the first surface relief feature 8 along its propagationpath, a portion of its rays obtains a greater out-of-plane angle thuswidening the angular distribution of light, as exemplified by a fan ofrays 804. In order to enable this widening of the angular distribution,face 16 of feature 8 is inclined at low slope angle a with respect toprevailing plane 202 of surface 10.

The slope of face 16 is sufficiently small and 90°−α<<ϕ_(TIR1). Thisprevents refraction at face 16 and light escape from waveguide 4 throughsurface 10. Therefore, the interaction of light propagating in waveguide4 with face 16 will result in TIR back into the waveguide. Moreover, theslope angle α is also low so that the increment in the angulardistribution it produces is substantially less than the angular span offan of rays 802.

Face 16 has a substantially planar shape and smooth surface.Accordingly, light rays striking face 16 will obtain an increment intheir out-of-plane angles which is twice the slope angle α. It will beappreciated that the range of directions represented by fan of rays 802is at least partially overlapping with the range of directionsrepresented by fan of rays 804.

As a result of TIR from face 16 of an first feature 8, at least some ofthe uttermost rays in fan of rays 804 may form incidence angles withrespect to a normal to surface 12 greater than second TIR angleϕ_(TIR2). Therefore, upon reaching surface 12, these uttermost rays willcross said surface and refract into buffer layer 6, as illustrated by afan of rays 808. It will be understood that this escaping lightrepresents a small portion of light guided through waveguide 4 and isgenerally characterized by relatively low emergence angles in layer 6(or high refraction angles with respect to a surface normal).Furthermore, it will be understood that, when angle α is sufficientlylow, the angular span of fan of rays 808 will also be relatively low.Moreover, the divergence of fan of rays 808 can be easily controlled byvarying slope angle α of the respective face 16. For example, when ahigh degree of light collimation is desired, the fan of rays 808 may beprovided with a very low divergence by making angle α very low,accordingly.

As the extracted light emerges from the core of waveguide 4, it furthercrosses buffer layer 6 and eventually strikes its outer surface 14.Surface 14 representing the optical interface between layer 6 and lightextracting layer 20 is configured for an unimpeded light passage intolayer 20. Particularly, the inner layer 106 of the light turning filmexemplifying layer 20 may be provided with the refracting indexapproximately matching the refractive index of layer 6, in which caseTIR and Fresnel reflections can be substantially suppressed.

Referring further to FIG. 10 , a fan of rays 810 represents light whichis extracted from waveguide 4 and which further propagates into layer106. Obviously, when the refractive indices of layers 6 and 106 arematched, fan of rays 810 can have essentially the same narrow angularspan and low slope with respect to the prevailing plane of system 2 asfan of rays 808.

Referring yet further to FIG. 10 , the corrugated boundary 154 betweenlayer 106 and 104 comprises a plurality of asymmetric corrugations 108defined by two different slopes of the opposing facets forming each ofsaid corrugations. As explained in the above examples, corrugations 108redirect light propagating at low angles towards a normal to theprevailing plane of system 2, as illustrated by a fan of rays 812.

It will be appreciated that fan of rays 812 may have a slightlydifferent angular span than fan of rays 810 due to the at least onerefraction occurring at boundary 154. However, it will also beappreciated that corrugations 108 may be designed to result in theangular span of fan of rays 812 still being sufficiently narrow.

As light redirected by the light turning film of layer 20 exits fromsystem 2 along a normal to surface 110, it remains confined within arelatively narrow angular cone, as illustrated by a fan of rays 814.When exiting from surface 110, the out-of-normal rays may undergo somerefraction further away from the surface normal since the refractiveindex of the outside medium is lower than that of layer 104. However,when the angular distribution of fan of rays 812 is sufficiently narrow,the angular distribution within the emergent fan of rays 814 will alsobe relatively narrow.

Thus, system 2 can emit highly collimated light from its frontal surfacewithout employing traditional collimating elements such as lenses ormirrors. Accordingly, it can be shown that the process of lightextraction and collimation can continue along the light propagation pathin waveguide 4. As illustrated by a fan of rays 806 representing lightpropagating at greater-than-TIR angles and reflected from surface 12,layer 6 depletes light from waveguide 4 in a controlled manner by“shaving-off” only a narrow cone of the uttermost rays. The raysescaping into buffer 6 obtain their sub-TIR angles with respect tosurface 12 due to TIR from features 8. Since features 8 are distributedalong the longitudinal axis 200 of waveguide 4, they will continueproviding additional angular bias to the guided light and thus result incontinuous light extraction from system 2 through surface 110.

In view of the above description, it will be appreciated that thecollimating function of system 2 was achieved using simple,non-collimating and non-focusing elements for light-deflection, such asshallow surface microstructures having planar surfaces. The prior-artdevices used for directing light into a relatively narrow emission coneordinarily use various complex-shape collimating optical elements suchas spherical or aspherical lenses, parabolic or spherical mirrors, aswell as arrays of such optical elements in various combinations. Incontrast, the above illustrated embodiments of system 2 use no suchcomplex shapes of elements.

Furthermore, the conventional devices employing lenses, mirrors or theirarrays and commonly require precise positioning of the light emittingfeatures with respect to the collimating elements. Particularly, eachlight emitting feature should typically be positioned along the opticalaxis and in focal area of the respective collimating element. Contraryto that, the layers or individual light deflecting or redirectingfeatures of system 2 do not necessarily require any special positioningor alignment with respect to each other except the very basic alignmentor positioning of the layers with respect to each other. Thus, therelatively simple and manufacturing-friendly structure of system 2 canbe advantageously selected for a number of illumination applicationsrequiring at least some degree of collimation, such as, for example,directional wide-area illuminators, LED panel luminaires for general orspecial lighting, spotlights, accent lights, flashlights, backlightswith a limited emission angle, and the like.

FIG. 11 illustrates an alternative light-collimating variation of system2 in which buffer layer 6 has corrugated external surface 14 and lightextracting layer 20 is disposed in contact with layer 6 conforming tothe relief of surface 14. Additionally, light extracting layer 20 has ahigher refractive index than layer 6 so that the pair of layers 6 and 20forms a light turning structure similar to the light turning filmdescribed in the above examples. Accordingly, corrugations 108 are nowformed by the corrugated boundary between layers 6 and 20.

Each corrugation 108 includes facet 124 configured for refracting lighttowards surface 110 and adjacent facet 126 configured for reflectinglight by means of TIR generally along a normal to surface 110. Thus,facets 124 and 126 may be configured to provide nearly 90° light bendingby two stages: the first stage being the refraction at facet 124 and thesecond stage being TIR at facet 126. It will be appreciated that theslope of facets 124 may be advantageously selected to intercept and bendsubstantially all of the light escaping from waveguide 4 into bufferlayer 6.

Similarly to the above-described embodiments and examples, since bufferlayer 6 has a lower refractive index than the core of waveguide 4, itprovides the required asymmetry in refractive indices at the opticalinterfaces formed by surfaces 10 and 12 so that light escapes fromwaveguide 4 primarily through layer 6 and the light-turning structureformed by layers 6 and 20.

Accordingly, each face 16 of surface relief features 8 formed in surface10 has slope angle α which is low enough to prevent light leakagethrough surface 10 but is sufficient to eventually extract at least asubstantial part of light propagating in waveguide 4. As explainedabove, this requirement may be generally satisfied by limiting angles αto less than ϕ_(TIR2)−ϕ_(TIR1), where ϕ_(TIR1) and ϕ_(TIR2) are the TIRangles at the waveguide 4 boundaries formed by surfaces 10 and 12,respectively. Furthermore, angle α may be further restricted to evensmaller angles to minimize the fan-out angle of the light escaping intolayer 6 and/or reducing or eliminating the unwanted light leakageresulting from Fresnel reflections at surface 12.

It will be appreciated by those skilled in the art that the Fresnelreflection generally occurs at each light passage from one refractivemedium into another if there is a difference in refractive indicesbetween the media. Although the Fresnel reflections usually account fora small fraction of light energy refracting into the other medium,especially when the difference of refractive indices is relativelysmall, the relative amount of reflected light increases at highincidence angles. Particularly, Fresnel reflection increases when lighttravels from a higher refractive index medium into a lower refractiveindex medium at an angle of incidence closely approaching the TIR angleat the optical interface between the two media. Therefore, referring tothe optical interface formed by the core of waveguide 4 and buffer layer6, some light may still reflect from surface 12 back into the waveguidecore even when the incidence angle is lower that the second TIR angleϕ_(TIR2).

In order to minimize the chance for such rays to exit through surface10, angle α may be limited to a reduced allowable angular range of0<α<<ϕ_(TIR2)−ϕ_(TIR1). In this case, sub-TIR rays reflected fromsurface 12 back towards surface 10 by means of Fresnel reflection willstrike the respective face 16 at an incidence angle which is still lessthan ϕ_(TIR1). Accordingly, the respective face 16 will reflect saidrays towards surface 12 by means of TIR which will prevent prematurelight escaping from waveguide 4 through surface 10 and still result inlight decoupling from surface 12. Thus, the sufficiently low angles αmay provide a sufficient cushion for light recycling in waveguide 4 andmaintaining its intended operation even in the presence of unwantedreflections from surface 12.

Referring further to FIG. 11 , the light exit portion of system 2 may beprovided with a light diffusing layer. For example, the external surface110 of layer 20 may be patterned to provide diffusing properties.Alternatively, a light diffusing film or sheet may be attached tosurface 110. Such diffusing layer may be used to soften the angulardistribution of collimated light emitted by system 2. It may also have afunction of masking the intensity irregularities across thelight-emitting surface. The surface features of the light diffusinglayer may be configured to limit the diffusion angle to a desired beamspread and generally preserve the directionality of the emitted beam.

FIG. 12 shows an embodiment of system 2 in which layer 20 has amicrostructured surface configured for extracting light from system 2 bymeans of refraction and/or TIR. The microstructured surface of layer 20represents a linear prism array where each linear prism extendsperpendicular to axis 200 and generally parallel to linear surfacerelief features 8 of waveguide 4.

Similarly to the embodiment of FIG. 6 , light source 400 provided on thewaveguide's light input edge illuminates the edge and injects light intowaveguide 4. The light injected into the waveguide propagates in awaveguide mode by bouncing from opposing surface 10 and 12 of thewaveguide. The repetitive pattern of surface relief features 8 alongaxis 200 ensures that most light rays undergo multiple interactions withfeatures 8 along the propagation path. Surface relief features 8 deflectlight from the original propagation direction by incrementallycommunicating the respective light rays a greater out-of-plane angle ateach interaction eventually resulting in light decoupling from waveguide4 into buffer layer 6. The dihedral angles of the shallow prismaticcorrugations representing features 8 are so selected as to result inextracting at least a substantial part of light from waveguide 4 bymeans of incremental deflections.

Layer 20 preferably having the same or greater refractive index thanlayer 6 receives light emerging from waveguide 4 and layer 6 and furtherredirects it out from system 2. For this purpose, the facets of eachlinear prism of light extraction layer 20 should be positioned toprevent light reflection back into layer 20. In one embodiment, suchsystem 2 may be utilized as a broad-area luminaire emitting light at anangle with respect to a surface normal. In one embodiment, such system 2may be utilized as a front-light in which case, for example, an imageprint or painting (not shown in FIG. 12 ) may be externally attached tolayer 20 or disposed in an immediate proximity to said layer.

Each linear prism of layer 20 may also be configured to intercept lightrays propagating at a first angle with respect to a surface normal andredirect them at a greater angle with respect to the same normal so thatlight emitted by system 2 is collimated at least in a planeperpendicular to the longitudinal axis of the array of prisms. Asillustrated in FIG. 12 by example of ray 74, at least some rays may beemitted from system 2 in a perpendicular direction with respect to thesurface plane.

The prismatic layer 20 may be made by a variety of means. In anon-limiting example such layer 20 may be made in the form of amicrostructured sheet or film and then laminated onto layer 6.Alternatively, layer 20 may be deposited onto layer 6 first and theprismatic array may be embossed in a subsequent step. In a furthernon-limiting example, the fabrication of system 2 may include initiallyforming a complete layered structure (including waveguide 4 and layers 6and 20) with smooth external surfaces and subsequently providingmicrostructures in waveguide 4 and layer 20 in a single step.

FIG. 13 illustrates a front-light implementation of system 2 where layer20 comprises a screen having at least one light scattering surface to beilluminated by the light propagating in waveguide 4. Such layer 20 maybe formed, for example, by providing an opaque or semi-opaque scatteringlayer on top of surface 14. The opaque light scattering layer may beprovided by a variety of means and may include, for example, white paintor pigment, colored paint or pigment, back-scattering film, surfacetexture, ink, phosphorescent or fluorescent substance, liquid crystals,etc. In a further non-limiting example, layer 20 may comprise a screencontaining any print, image, logo, text, symbols, pattern, or the likefeatures. When the screen includes an image print, the print may beformed directly on surface 14 using screen printing, digital printing,offset printing, ink spraying, hand painting, machine painting or thelike processes. Alternatively, the print may be formed on the surface ofan external film, paper or any other suitable substrate which can belaminated or otherwise attached to surface 14. Such external film can bemade of an opaque material in which case the printed surface should facelayer 6. Alternatively, the external film with an image print can bemade from an optically transparent material in which case the print mayface either layer 6 or away from layer 6.

Referring further to FIG. 13 , rays 72 and 76 having sufficiently highout-of-plane angles exit from waveguide 4 into buffer layer 6 atdifferent locations along axis 200 where they are scattered by layer 20generally towards a normal to axis 200. In contrast, ray 72 illustratingthe main bulk of light rays propagating in waveguide 4 at lowerout-of-plane propagation angles, continues to be guided by means of TIRfrom surfaces 10 and 12 until it obtains the sufficient out-of-planeangle to exit into layer 6 and to be extracted from system 2 by layer20. Layer 20 may be configured to scatter light primarily towardssurface 10 and may optionally incorporate a reflective layer to reflectany stray light escaping towards the opposing direction.

According to at least some embodiments, the slope of the lightreflecting faces 16 of surfaces relief features 8 can be made low enoughin order not to perceptibly affect the visual appearance of surface 10or the light-scattering screen of layer 20 compared to the case whensurface 10 is perfectly smooth and flat. Additionally, the surfacedepressions forming features 8 may be made substantially shallow so asnot to significantly deflect light rays propagating at low angles withrespect to a normal to the prevailing plane of system 2.

FIG. 14 further illustrates the operation of system 2 of FIG. 13 , wheresystem 2 has a planar front-light configuration. For the purpose ofillustrating the front-light operation of system 2, it is assumed thatan observer is viewing system 2 from a normal viewing angle.

Obviously, each sloped face 16 will slightly alter the light propagationpath between the viewer and the light scattering screen compared to thecase where surface 10 would be perfectly smooth and planar. Slope angleα of faces 16 with respect to plane 202 of surface 10 will define howmuch light will deviate from “an ideal” path along the surface normal.Accordingly, at any non-zero angle α, an actual light path 402 from alight emitting/scattering point at surface 14 to a viewer's eye 660 willbe different from a hypothetical light path coinciding with a normal 800to plane 202. Particularly, path 402 will deviate by a deviation angle δfrom normal 800 and result in the observer viewing a different area oflayer 20 which is offset from the respective “on-axis” area by an offsetdistance 406. This offset distance 406 depends on angle α, as well as onthe refractive indices and thicknesses of waveguide 4 and layer 6. Iflayer 20 comprises a high-fidelity image print and angle α is high, theobserver may experience seeing the neighbouring image pixels compared tothe case of viewing the same print through a perfectly flat transparentplate.

However, it will be appreciated that angle α may be selected to besufficiently low so that deviation angle δ will also be low resulting ina negligibly small offset distance 406 so that the observer will notexperience a perceptible change in the visual image quality. By way ofexample and not limitation, angle α may take particular values of 1angular degree or less. It can be shown that at such slope angles offaces 16 and with using some common transparent materials for waveguide4 and buffer layer 6, the deviation angle δ will also be about 1 degreeor less, in which case the offset distance 406 will generally not exceed1.5-2% of the combined thickness of waveguide 4 and layer 6.Particularly, if the thickness of the respective transparent layers ofsystem 2 is about 5 mm, offset distance 406 will generally be less than100 microns at near-normal viewing angles.

Furthermore, the slopes of faces 16 can be made identical to each otherin which case the light path deviations caused by the plurality ofindividual features 8 will simply translate the entire image, as viewedby the observer, perpendicularly to the surface normal by a smalldistance and thus will also not cause the loss of perceptive imagefidelity. In the illustrated front light configuration of system 2,faces 18 can be made perpendicular or near-perpendicular to surface 10so that the visible aperture of faces 18 will be negligibly small, alsobeing substantially smaller than the visible aperture of faces 16. Thisshould ensure that faces 18 do not notably interfere with image viewing.

As substantially all of the light propagating through system 2 can bedistributed along waveguide 4 and emitted towards the image print alongthe propagation path, the efficiency of system 2 as a front light can bemade fairly high. As explained above, surface relief features 8preliminary extract light into buffer layer 6 where the extracted lightilluminates layer 20. Layer 20, in turn, scatters light towards theobserver and permanently extracts at least a substantial portion oflight from system 2. Since the unwanted light leakage through surface 10is eliminated or at least substantially reduced by providingsufficiently low slopes of faces 16 and by providing an asymmetry inrefractive indices of the media adjacent to surfaces 10 and 12 ofwaveguide 4, a front light employing system 2 may be used for displayingimages in higher fidelity, bright illumination, and improved contrastcompared to conventional edge-lit front lights.

FIG. 15 illustrates a variation of light scattering layer 20 which isconfigured to scatter light into both hemispheres from its plane. Suchlayer 20 may be exemplified by a textured matte-finish surface of anoptically transmissive plate or film. Such layer 20 may also beexemplified by a semi-transparent layer of white or colored paint, ink,phosphors or dye deposited onto an optically transmissive surface. Theappropriate light scattering features of layer 20 may be formed directlyon surface 14 or they may be provided on a transparent or translucentsubstrate which can be attached to surface 14.

In a backlight variation of this invention, layer 20 may be providedwith light diffusing features which diffuse and forward-scatter lightemerging from layer 6 toward the viewer. By way of non-limiting example,the light diffusing layer 20 of FIG. 15 can be configured to provide abright uniform glow from its surface. It may also be associated with atranslucent image print, see-through LCD screen, colored film and thelike, making system 2 suitable for edge-lit signage and generalillumination applications.

FIG. 16 shows an embodiment of system 2 in which surface relief features8 are exemplified by smooth and shallow linear undulations orcorrugations extending generally perpendicular to longitudinal axis 200of waveguide 4 and providing surface waviness in a cross-sectionparallel to axis 200. In the illustrated embodiment, faces 16 and 18both have smooth curved surfaces which smooth conjugates between eachother.

Similarly to the sharp-corner surface relief features 8 discussed above,the smooth linear undulations may be formed by casting or extrusion ofwaveguide 4 from an optically transmissive polymeric material or formedin a flat sheet of glass or polymer by micro-replication or materialremoval. In another example applicable to both the planar andcylindrical geometries of waveguide 4, the smooth undulations orcorrugations of surface relief features 8 may be formed by laserablation or thermal evaporation. In the case of waveguide 4 made fromacrylic, a CO₂ laser with the operating wavelength of about 10 micronsmay be used to selectively ablate the surface material and produce therequired features. Optional polishing may include, for example, buffing,flame polishing or thermal annealing. In further examples, the smoothsurface 10 may be subjected to any other suitable surface modificationprocess such as embossing, imprinting or etching in order to produce thesuitable surface relief features 8. In yet further examples, variousprocesses involving heat sources may be used to modify surface 10accordingly by means of material melting softening, thinning,stretching, etc.

The surface undulations may be made periodic and having a constant pitchand/or slope. Alternatively, the width or slope of each undulation maybe made variable in a cross-section along the propagation path. Theamplitude of the undulations may also be made constant or variable.Particularly, if a constant pitch is employed, the amplitude or surfaceslope may be made increasing along the propagation path in order tocompensate the gradual light depletion in waveguide 4. A usefulvariation of surface relief features 8 may include shallow surfaceundulations having a variable slope of faces 26 which increases alongthe intended optical path. It will be appreciated that the increase ofthe slopes of undulations or corrugations along waveguide 4 willincrease the rate of light extraction from the waveguide along theoptical path thus compensating the light depletion and resulting in animproved uniformity across the waveguide's surface. Particularly, theindividual slopes of faces 16 may be selected to provide lightuniformity within 20-30% across the light emitting surface of system 2.

Furthermore, undulations or corrugations forming features 8 may be madeessentially random within predefined ranges of width, height and/orslopes. The distribution of surface relief features 8 along axis 200 mayalso be made random or ordered. The randomization or quasi-randomizationof features 8 may have a particular advantage for simplifying thefabrication process as well as for reducing the glare from surface 10 inthe end products employing system 2. The light extracting properties ofwaveguide 4 essentially equivalent to making smooth undulations insurface 10 may also be provided by making the thickness of waveguide 4variable along the propagation path.

Accordingly, each surface relief feature 8 represented by smooth surfaceundulations or corrugations may be configured to have a reflective face16 facing the light source and an opposing face 18 facing away from thelight source. Each face 16 may be shaped so that at least a portion ofits surface is generally inclined at the appropriate angle α withrespect to the prevailing plane 202 of surface 10. In FIG. 16 , slopeangle α is illustrated by the angle between plane 202 and a tangent 30to face 16 of surface relief feature 8. Similarly to at least some ofthe above described embodiments, the acceptable range of angle α may beselected from the following relationship: 0<α<ϕ_(TIR2)−ϕ_(TIR1).

Each face 16 is designed to introduce an additional out-of-plane angleto light propagating in waveguide 4 and extract light propagating atnear-critical TIR angles into layer 6. Layer 20 is provided to finallyextract light from system 2 and scatter or direct the extracted lightout of the illumination system.

Also, in a continuing similarly to the above described embodimentsemploying sharp-cornered shallow depressions and planar faces 16 and 18,the depth of each corrugation or undulation forming features 8 can bemade sufficiently small relatively to the width so as to result in verylow slope angles that faces 16 and 18 make with plane 202. On one hand,it allows for distributing light along a considerable length ofwaveguide 4 since each feature 8 redirects only a small fraction oflight propagating in waveguide 4 and extracts only a portion of lightstriking its surface allowing the rest to be guided further through thewaveguide by means of TIR. On the other hand, low slope angles providefor low deviation angles δ and small offset distances 406 for theviewer, which makes system 2 particularly suitable for low-distortionedge-lit front lights.

FIG. 17 shows an embodiment of waveguide illumination system 2 in whichan external cladding layer 22 is provided on top of surface 10. Layer 22may be made from the same or similar material that buffer layer 6 andmay have an external boundary 122 with the outside medium such as air.Cladding layer 22 may provide protection of surface 10 from abrasion,scratches, contamination or optical contacting with other bodies orsubstances which may adversely impact the light guiding properties.Ordinarily, the refractive index of cladding layer 22 should not exceedthe refractive index of buffer layer 6 to provide for maximum isolationof light in waveguide 4. However, it should be understood that layer 22may also have any other suitable refractive index. It will beappreciated that even when the refractive index of layer 22 is the samelower than that of layer 6, system 2 may still operate in the mannerdescribed above. Although at least some of the light rays propagating inwaveguide 4 and obtaining angles lower than critical TIR angle ϕ_(TIRC)with respect to the surface normal may transiently exit into layer 22,the boundary 122 of layer 22 with the even lower-index outside mediumwill ensure that these rays will reflect from surface 122 by means ofTIR. The rays reflected from surface 122 can then return back towaveguide 4 where they can ultimately escape through buffer 6 and can befurther extracted from system 2 by layer 20.

FIG. 18 illustrates an embodiment of waveguide illumination system 2where system 2 has a cylindrical configuration. By way of example,waveguide 4 may be represented by a large-core polymer optical fiber(LCPOF). LCPOF may also include an optional cladding layer (not shown inFIG. 18 ) surrounding the fiber core. Surface relief features 8 areformed in the side of the cylindrical waveguide 4 opposite to theintended light emission direction. The opposing side of cylindricalwaveguide 4 is provided with buffer layer 6 having the refractive indexlower than the refractive index of the fiber's core. Light extractionlayer 20 is provided on top of layer 6 and may comprise, by way ofexample, a light turning film designed to accept light emerging at lowangles from the fiber and redirect it at a higher angle with respect tothe fiber's longitudinal axis. Each feature 8 may have face 16 facingthe light source and adjacent face 18 facing away from the light source.The slope of each face 16 with respect to the longitudinal axis 200 ofwaveguide 4 is sufficiently low in order to prevent light escape throughthat face and in order to cause the extraction of relatively smallportions of light into layer 6 along the propagation path according tothe principles discussed above.

In operation, light rays initially propagate in the fiber's core ofwaveguide 4 at propagation angles permitting for TIR from thelongitudinal walls of the fiber, that is at the incidence angles withrespect to a surface normal greater than critical TIR angle ϕ_(TIRC).Each ray having a sufficient out-of-plane propagation angle eventuallystrikes one or more faces 16 which progressively communicate greaterout-of-plane propagation angles to the ray at each interaction. As anyray reaches the minimum out-of-plane angle sufficient for suppressingTIR at the boundary with buffer layer 6, it can escape into layer 6 andcan be further directed by the light turning film of layer 20.Particularly, the light turning film may be configured to emit light ina relatively narrow range towards a perpendicular to the fiber's axis atleast in a cross-sectional plane parallel to said axis. Additionally,since the fiber ordinarily has a circular or elliptical transversalcross-section, the cylindrical configuration of the waveguide 4 may alsoprovide at least some light collimation in the plane perpendicular tothe longitudinal axis 200. Therefore, it will be appreciated that system2 having a cylindrical configuration may be configured to collimatelight in one or two dimensions and emit the collimated lightperpendicular to the fiber along its entire length thus providing anefficient side-emitting fiber illuminations system.

It should be understood, however, that the application of cylindricalconfigurations of waveguide 4 is not limited to the side emitting fibersbut also includes various light pipes, edge illuminators or any suitableillumination systems which may benefit from the elongated shape of thelight distributing waveguide. In a cylindrical configuration, waveguide4 may have any suitable shape in a cross-section perpendicular to thelongitudinal axis 200 of the waveguide. Suitable cross-sectional shapesmay include but are not limited to: circular, elliptical, square,rectangular, hexagonal, trapezoidal or other shapes having any number ofsides each having straight or curved profiles. The cross-sectional shapemay also be formed by a profile which contour can be made variable alongthe longitudinal axis of waveguide 4.

FIG. 19 depicts an embodiment of waveguide illumination system 2illustrating the incremental increasing the out-of-plane angle of theguided light by multiple TIR reflections from respective surface relieffeatures 8. Referring to FIG. 19 , a light ray emitted by edge-coupledlight source 400 initially propagates in waveguide 4 at an incidenceangle with respect to surface 12 far exceeding the critical TIR angleϕ_(TIRC). Surface relief features 8 formed in surface 10 are distributedalong the longitudinal axis of the waveguide.

The plurality of features 8 alters a generally planar cross-sectionaloutline of surface 10. Each feature 8 is formed by a shallow recess ordepression in surface 10 and has two opposing adjacent faces, 16 and 18.Face 16 is facing the light source and has a low slope angle withrespect to the prevailing plane of surface 10. Face 18 is facing awayfrom the light source and has a generally higher slope with respect tothe prevailing plane of surface 10.

As the light ray randomly encounters features 8 on its path, it strikesthe respective faces 16 and reflects from them by means of TIR. Sincethe angle of reflection is equal to the angle of incidence with respectto a normal to face 16, the ray incrementally obtains a greaterout-of-plane angle at each interaction with features 8 and continues topropagate along the longitudinal axis of waveguide 4. This process iscontinuing until the angle of incidence to surface 12 exceeds the TIRangle at that surface in which case the ray escapes into buffer layer 6.

It will be appreciated that, depending on the initial propagation angle,light may propagate different distances in waveguide 4 until it exitsinto buffer layer 6, even if the slopes of reflective faces are keptconstant. Considering that the conventional light sources have at leastsome beam divergence, at least a substantial portion of light inputthrough the waveguide's edge can be effectively distributed along thelongitudinal axis of waveguide 4 and extracted from surface 12 atgenerally low emergence angles with respect to that surface.

As the light ray decoupled from the core of waveguide 4 furtherpropagates through layer 6, it reaches light extracting layer 20. Thelight turning film of layer 20 intercepts light emerging from waveguide4 and redirects it at a normal angle with respect to the prevailingplane of system 2. Thus, light becomes effectively extracted from system2 with collimation. This operation makes system 2 particularly suitablefor making directional illumination systems, such as, for example,side-emitting large-core fibers and planar edge-lit LED panels.

Various parameters of surface relief features 8 may be varied to finetune the light distribution and emission from the surface of waveguide4. These parameters include but are not limited to: width, height orslope of reflective faces 26, general shape and distribution of features8 along the propagation path, etc. An optional mirrored surface may beprovided along surface 10 of waveguide 4 to reflect any stray light thatmay escape from the waveguide towards a direction opposing to layer 20.

It should be understood that the differential between the stepped dropin refractive indices outwardly at surface 10 and 12 of the waveguide 4is important to force light to escape from the waveguide's coregenerally through surface 12 and not through surface 10. As illustratedabove, such differential can be easily obtained by providing bufferlayer 6 having a lower refractive index than the core of waveguide 4 buthigher than that of the outside medium. Since the addition of the bufferlayer can generally lower the acceptance angle of waveguide 4 comparedto the bare waveguide core surrounded by low-n air on both sides, alight collimating feature may be associated with light source 400 orwith the light input edge of the waveguide in order to narrow thenatural divergence of light beam emanated by the light source.

FIG. 20 shows a collimating element 440 attached to the light input edgeof waveguide 4. Element 440 can be made from transparent material andmay have any optical configuration suitable for coupling light fromsource 400 into waveguide 4 at a limited range of propagation anglespermitting for TIR from both surfaces 10 and 12. Element 440 may beattached to the light input edge using an optical adhesive.Alternatively, collimating element 440 may be provided as an integralpart of waveguide 4 and formed by tapering the light input edge ofwaveguide 4 accordingly. Collimating element 440 is preferablyconfigured to have opposing concave walls reflecting light by means ofTIR. However, particularly when the divergence of light from source 400is too high for TIR and/or when source 400 is coupled to the light inputedge using a refractive medium such as optical adhesive or encapsulant,the concave walls of element 440 may be mirrored for increasing theacceptance angle of the collimating element.

Considering that waveguide 4 will only effectively conduct light thatenters its edge within a certain acceptance cone, let's define anacceptance angle θ_(max) of waveguide 4 being the half-angle of thisacceptance cone. It will be appreciated by those skilled in the art thatacceptance angle θ_(max) can be found from the following expression:

${{\sin\theta_{\max}} = \frac{\sqrt{n_{1}^{2} - n_{2}^{2}}}{n_{0}}},$

where n₁ is the refractive index of the core of waveguide 4, n₂ is therefractive index of buffer layer 6 and n₀ is the reflective index of themedium light is traveling through before entering waveguide 4. Whensource 400 is coupled to the light input edge of waveguide 4 through alayer of air (n₀≈1), sin θ_(max)=√{square root over (n₁ ²−n₂ ²)}.Accordingly, a numerical aperture (NA) of waveguide 4 can be defined asNA=n₀ sin θ_(max), or, in the case of source to waveguide couplingthrough air, NA=sin θ_(max)=√{square root over (n₁ ²−n₂ ²)}.

As illustrated in FIG. 20 , collimating element 440 may be useful toinject far off-axis rays into waveguide 4 so that they can propagate bymeans of TIR in the waveguide's core and can also be distributed andemitted through surface 14 as explained in the examples above.Collimating element 440 should be preferably configured to provide atleast some initial divergence of light upon entering into waveguide 4.Particularly, when it is desired that system 2 can begin emitting lightwithin a short distance from the light input edge, collimating element440 may be configured to provide the initial divergence approximatingthe acceptance cone with the half angle being close to the acceptanceangle θ_(max) of waveguide 4.

FIG. 21 shows an embodiment of waveguide illumination system 2comprising light source 400, waveguide 4, buffer layer 6 and lightextraction layer 20. Waveguide 4 has a wedge shape with the light inputedge being wider than the opposing terminal edge and with the thicknessof waveguide 4 gradually decreasing along the light propagation path.The taper of waveguide 4 defined by slope angle α is relatively low sothat waveguide 4 still has a generally planar configuration. Surface 10is generally smooth and may have one or more flat portions and at leastone inclined surface portion defining an extended surface relief feature8 and its reflective face 16 facing the light guided in waveguide 4.Buffer layer 6 is provided on opposing side waveguide 4 and is attachedto surface 12 with a good optical contact. Layer 6 is further followedby light extraction layer 20.

The refractive index of buffer layer 6 is lower than the refractiveindex of waveguide 4 which creates a differential in the refractiveindex drop at surfaces 10 and 12 and enables the preference for lightescaping through surface 12 when the light is bent to sufficiently highout-of-plane angles. Light extraction layer 20 is exemplified by a lightturning film which turns light by almost 90 degrees so that the lightrays emerging from layer 6 at low angles with respect to surface 14 canbe directed generally towards a normal to the prevailing plane of system2. Layer 20 may also comprise a light scattering surface, screen, imageprint, etc., as discussed above.

Similarly to the above-described principles, light rays propagating in awaveguide mode and having different out-of-plane angles will emerge fromsystem 2 at different locations along the propagation path, depending onthe slope of face 16, resulting in light distribution and extractionalong the extent of the light emitting region of waveguide 4. It will beappreciated that slope angle α can be made sufficiently small (about onedegree or less) which will result in a small angular divergence of lightemerging into buffer layer 6. Accordingly, the light turning film oflayer 20 can be configured to turn light emerging from buffer layer 6 byup to 90° away from the surface plane while preserving the smalldivergence. As the light beam turned by the light turning film overcomesTIR and emerges from layer 20, it will have a well defineddirectionality and due to being confined within a finite angular range.Thus, system 2 depicted in FIG. 21 can be configured to emit light fromthe entire light emitting region with improved light collimation.

FIG. 22 illustrates a front-light variation of system 2 employing awedge-shaped waveguide 4 in which layer 20 comprises a screen thatback-scatters light generally towards a normal to the prevailing planeof waveguide 4, so that at least a substantial part of the scatteredlight can pass through waveguide 4 and exit from system 2 throughsurface 10. The screen of layer 20 may be formed by a bright, lightscattering paint or film for illumination purposes. Alternatively, thescreen may comprise an image print viewable through layer 6 andwaveguide 4 and deposited onto surface 14 or onto an intermediatesubstrate using any suitable printing process.

Referring to FIG. 22 , light can propagate considerable distances alonglongitudinal axis 200 in a waveguide mode while undergoing multiple TIRinteractions with opposing surfaces 10 and 12. Upon each interactionwith a sloped portion of surface 10, light is progressively communicateda broader angular spread with respect to axis 200, as illustrated by theexample of ray 72. Upon reaching a sub-TIR angle with respect to surface12, at least the uttermost out-of-plane rays, as exemplified by rays 76and 72, will exit from the core of waveguide 4 at different locationsalong axis 200, depending on the initial propagation angles.Accordingly, light extraction layer 20 extracts the emergent light bymeans of scattering which causes at least portions of the scatteredlight to overcome TIR at surface 10 and exit from system 2 towards anormal to the surface.

FIG. 23 illustrates a further variation of system 2 in which layer 6 hascorrugated outer surface 14 and layer 20 conforms to this corrugatedshape thus also forming a corrugated boundary between the two layers.Additionally, light extracting layer 20 has a specularly reflectivesurface which can be obtained, for example, by mirroring surface 14 orby depositing a specularly reflective film or foil onto layer 6. Thecorrugations (features 18) can be made identical to each other andhaving a constant pitch.

Referring to FIG. 23 , facets 124 facing the light source may beadvantageously configured to reflect light emerging from waveguide 4 atangles approximately perpendicular to the prevailing plane of system 2,as illustrated by ray 74, thus providing an efficient light turningstructure of a reflective type. According to the principles discussedabove, waveguide 4 may be configured to emit light into buffer layer 6within a narrow angular cone. In turn, such reflective light turningstructure may be configured to redirect the emitted light towardssurface normal while preserving the narrow beam divergence. It will beappreciated that this will result in system 2 emitting a highlycollimated beam from surface 10.

It will be understood that system 2 may include any other variations oflight extracting layer 20 which may be configured to permanently extractlight from system 2 and direct and distribute it according to thespecific application. Light extraction layer 20 may also incorporate anyother light directing structures which change the propagation path oflight emerging from waveguide 4 at low angles with respect its majorsurface. Various modifications of layer 20 may include lens arrays,prism arrays, mirrors, diffusers, retroreflective elements, scatteringelements, color changing elements or layers, etc.

FIG. 24 explains the light collimation function of certain embodimentsof system 2 in further detail. In FIG. 24 , light source 400 emits lighthaving a relatively broad angular distribution 900. When such lightenters the light input edge or end of waveguide 4, it propagates withinthe waveguide along its longitudinal axis 200 undergoing multiplereflections from surfaces 10 and 12 by means of TIR. The interactions oflight with surface 10 include reflections from surface relief features 8(not shown) which gradually increases the out-of-plane angle of lightpropagation and eventually results in light escape from waveguide 4 intolow-n buffer layer 6.

As explained above, when the characteristic slope angle α of surfacerelief features is sufficiently low, the angular distribution of lightescaping into layer 6 is also very narrow as it generally subtends anangular range from zero to a small angle which depends on slope angle α.Accordingly, when light extraction layer 20 turns the emerging light byup to 90 degrees, the angular distribution of light emitted from thebroad surface of system 2 will also be relatively narrow. It will beappreciated that the light beam emerging from layer 20 may experiencesome broadening of the angular distribution compared to its propagationin the bulk materials of system 2 generally having refractive indicesconsiderably greater than a unity. Nevertheless, it will also beappreciated that the angular distribution of light emitted by system 2can be made substantially narrower than that of source 400.

This is illustrated in FIG. 24 by reference to angular distributions912, 914, 916, and 918 exemplifying collimated light emerging from thebroad-area surface of system 2 at different distances from source 400.Each the angular distributions 912, 914, 916, and 918 may be madenarrower than the broad distribution 900 of light source 400 at least ina plane in which the reflection by surface relief features 8 occurswithin waveguide 4. Therefore, system 2 may be configured to not onlydistribute light from a compact source over a large area and emit suchlight from said large area, thus resulting in a reduced glare, but alsoprovide an improved collimation and/or directionality of the emittedlight compared to the light beam emitted by the source.

It will be appreciated that the light-collimating embodiments of system2 illustrated above do not generally require applying any opaque ormirror layer onto surface 10 in order to prevent light escaping throughthat surface. Due to the combined function of surface relief features 8,which provide incremental out-of-plane deflection of light rayspropagating along axis 200, buffer layer 6, which preliminary extractsonly the uttermost deflected rays from waveguide 4, and light extractinglayer 20, which finally extracts the pre-extracted light from system 2,virtually no light may be allowed to exit through surface 10 even thoughsaid surface can have a very high optical trasmissivity.

This is in a sharp contrast to the conventional illumination systemsemploying a waveguide, such as edge-lit backlights or lightingluminaires, for example. In such conventional systems, a substantialportion of light escapes through an unwanted side of the waveguide(usually at least 25% and up to 50%) which requires using a specialdiffuse or specular reflector to be attached to that surface. Thereflector typically includes a sheet of highly reflective material whichredirects (with some reflection loss) the escaping light towards theother side of the waveguide. It will be appreciated that the use of anopaque reflector layer introduces additional losses (compared to TIR)and precludes the possibilities of transmitting light in a transversaldirection or using the system as a front light.

On the contrary, according to at least some embodiments of the presentinvention, waveguide illumination system 2 can maintain high transversaltransmissivity and allow for a generally unimpeded light passage alongin a perpendicular direction with respect to its major surfaces.Therefore, such system 2 may be used for transmitting light from adifferent light source in a transversal direction.

By way of example and not limitation, system 2 can be implemented as alight-collimating edge-lit luminaire which may also be positionedhorizontally in the light path of a daylighting system, such as askylight located above the luminaire. In such configuration, system 2can be configured to provide illumination by distributing and emittinglight emanated by one or more LEDs attached to the edge of waveguide 4and, additionally, to transmit light from the skylight perpendicularlythrough its body thus forming a combined solar/electric lightingluminaire.

FIG. 25 illustrates an exemplary embodiment of system 2 which is used atthe light exit end of a skylight 502. Skylight 502 can be ordinarilydesigned for rooftop installation and illumination of the buildinginterior with the natural sunlight through an opening in the roof orceiling underneath the skylight's light-collecting aperture.

Referring to FIG. 25 , skylight 502 includes a dome-shaped diffusersheet 260, and optional reflective side walls 512 and 514. Diffusersheet 260 can be made from an optically clear plastic material and mayhave at least one microstructured surface to improve light diffusion.Walls 512 and 514 are preferably covered with a sheet or film ofsecularly reflective material to aid in light channeling from sheet 260to the light-emitting opening below. The light emitting opening ofskylight 502 may have approximately the same aperture as thelight-receiving aperture of sheet 260. Alternatively, skylight 502 maybe made in a tapered version, in which case the above apertures may havedifferent transversal dimensions with respect to each other.

Referring further to FIG. 25 , waveguide illumination system 2 isimplemented in the form of a, optically transmissive planar panel andincludes waveguide layer 4, low-n buffer layer 6 and light extractinglayer 20 formed by a light turning film. Light source 400 including astrip of high-brightness LEDs is positioned adjacent to the light inputedge of waveguide 4. At least a substantial portion of surface 10 isprovided with sufficiently shallow surface relief features 8 eachincluding low-slope face 16 and adjacent face 18. The orientation ofsurface relief features 8 is such that each face 16 faces source 400 andeach face 18 is turned away from source 400. System 2 having the abovelayered structure and the form factor of a relatively thin panel ispositioned below skylight 502 in the respective light-emitting openingof the skylight so that the prevailing plane of waveguide 4 extendsperpendicularly to a vertical axis 44.

In electric lighting operation, waveguide 4 receives light from thelinear array of LEDs at its light input edge and guides said light bymeans of TIR. Accordingly, light injected through the light input edgepropagates in a waveguide mode along axis 200 towards the opposingterminal end of waveguide 4. According to the principles describedabove, surface relief features extract light into buffer layer 6 alongthe propagation path. In turn, the light turning film of lightextracting layer 20 finally extracts light downwards. Optionally, system2 may be configured to provide a prescribed degree of collimation andemit directional light along vertical axis 44. Accordingly, since lightescape through surface 10 is minimized or eliminated, substantially allof the light emitted by source 400 and distributed along waveguide 4 isemitted into the building interior. With the exception of light which isabsorbed or scattered during propagation in waveguide 4 or layers 6 and20, practically no additional light is lost in the system. Importantly,no light is directed back towards sheet 260 which would otherwiseconstitute a major energy loss in the case when waveguide 4 would employconventional microstructures or other types of prior-art lightextraction features.

In daylighting operation, system 2 receives a diffuse beam of sunlightemerging from sheet 260 and transmits it further downwards through itsbody. Since all respective layers and surfaces of system 2 are opticallytransmissive, the sunlight transversally passes through the panelwithout undergoing substantial reflection, backscattering orattenuation. When additional diffusing of daylight is necessary or whenmasking the portions of skylight disposed above the light emittingopening is desired, system 2 may further comprise one or more lightdiffusing layers operating in the transmissive mode.

In view of the above-described operation within a skylight, it will beappreciated that system 2 can emit artificial light from its broad-areasurface and also doubles as a skylight luminaire by allowing thedaylight into the building interior and optionally providing enhancedlight diffusion.

It will be appreciated that a unique operation of system 2 is obtained,at least in part, by employing a two-stage light extraction mechanism.The first stage includes incremental light deflection by surface relieffeatures 8 and the second stage includes beam turning by lightextracting layer 20. Buffer layer 6 separates these stages from eachother and provides the functional differential in the refractive indexdrop at the opposing sides of waveguide 4. By employing these features,in combination, system 2 suppresses light extraction through theunwanted side of waveguide 4 and can be configured to emit lightprimarily through the designated side or surface. When desired, asillustrated above, it may also be configured to almost completelyshut-off light emission from the unwanted side despite being opticallytransmissive and allowing light to pass transversely through its body.

FIG. 26 shows an alternative arrangement of the layers around waveguide4, where surface 10 is configured to be substantially flat but surface12, instead, has surface relief features formed by smooth and shallowcorrugations or undulations. Accordingly, buffer layer 6 and lightextraction layer 20 adjacent to surface 12 may conform to the shallowrelief features of surface 12 or may, alternatively, have flat externalsurfaces.

Accordingly, the refractive index of buffer layer 6 being greater thanthat of the outside medium and lower than that of waveguide 4 providesthe functional difference between TIR angles ϕ_(TIR1) and ϕ_(TIR2) atsurfaces 10 and 12, respectively. Therefore, the incremental angularbias of light propagation caused by the undulations of surface relieffeatures 8 along the propagation path in the waveguide 4 will result inlight escaping primarily through surface 12 while surface 10 willcontinue to reflect substantially all light by means of TIR.

By way of example and not limitation, layer 20 of the embodiment of FIG.26 may be formed by a scattering layer or a print formed directly onsurface 14 of layer 6 or formed on a separate substrate which can belaminated onto surface 14. When laminating layer 20 onto surface 14, asoft roller may be used for pressing layer 20 against surface 14 inorder to fill the shallow depressions in surface 14 and avoid forming ofair gaps.

The amplitude of the undulations forming surface relief features 8 mayordinarily be very small so that the relief of layer 20 will bevirtually unnoticeable when viewed from a distance. Additionally, theshallow surface relief features 8 will deflect light propagating betweenthe visible surface of layer 20 and viewer's eye 660 by only a smallamount causing no perceptible visual distortions.

Ray 72 illustrates a light path in waveguide 4 at relatively lowout-of-plane propagation angles allowing for TIR at both surfaces 10 and12. Each TIR of ray 72 from face 16 of the respective feature 8 willgenerally result in a greater out-of-plane angle thus introducingadditional angular bias and widening the angular distribution of lightguided in waveguide 4. Accordingly, ray 72 may eventually obtain anincidence angle less than the TIR angle with respect to surface 12 whenstriking one of the successive faces 16 and exit into layer 6 where itwill be scattered by layer 20 towards the viewer. Ray 74 illustrates aray path of the light being extracted from system 2 as it alreadyobtained a sufficiently high angle with respect to the longitudinal axisof waveguide 4. As ray 74 exits from waveguide 4 into buffer layer 6, itstrikes the light scattering surface of layer 20 and can be directedtowards the viewer thus providing the illumination function of anedge-lit front light.

FIG. 27A through FIG. 27I illustrate various shapes of surface relieffeatures 8. In FIG. 27A, feature 8 is shown to include face 16 inclinedat a low slope angle α to the prevailing plane of surface 10 andopposing face 18 inclined at a substantially greater slope angle to saidplane. In FIG. 27B, face 18 is shown being perpendicular to theprevailing plane of surface 10. It should be understood that theillustrative case of FIG. 27B may also include variations of face 18having angles which are close to 90° but not exactly normal.Particularly, when the respective feature 8 is formed in surface 10 byreplication processes like injection molding, compression molding,embossing, or extrusion processes, face 18 may have a suitable draftangle of 3-4° with respect to a surface normal. Such draft angle may benecessary, for example, to facilitate mold release.

In FIG. 27C, surface relief feature 8 is shown to additionally comprisea flat top portion 860 which extends parallel to the plane of surface 10and may also be configured to reflect by means of TIR. In FIG. 27D thetop portion 860 has a curved shape in a cross-section.

FIG. 27E shows feature 8 in which faces 16 and 18 are disposedsymmetrically and have about the same slope angle with respect to theplane of surface 10. Such a symmetric configuration of features 8 may beselected, for example, for the case where waveguide 4 is illuminatedfrom both opposing edges or terminal ends.

In FIG. 27F, surface relief feature 8 is formed by a smooth undulationor shallow recess in surface 10, where faces 16 and 18 are formed by theopposing side surfaces of the undulation. FIG. 27G shows surface relieffeature 8 formed by a funnel-shaped shallow cavity in surface 10. Itshould be understood that a relatively small portion of face 16 facinglight source 400 (not shown in FIG. 27G) may have an arbitrary highslope angle. Such portion may include, for example, the small areaimmediately adjacent to the vertex of the funnel. However, it isgenerally preferred that at least a major portion of face 16 still hasrelatively low slope angle α with respect to the prevailing plane ofsurface 10.

It will be appreciated that there is a great variety of possible shapesthat can be used for surface relief features 8. Accordingly, anysuitable profile or any suitable perturbation or irregularity ofotherwise smooth and flat surface 10 may be used to form individualsurface relief features 8, including any ordered or random surfacerelief structures, bumps, recesses, grooves, corrugations, surfacewaviness, etc., provided that they can introduce the required additionalangular bias for light propagating in waveguide 4 along its path andcause controlled light leakage into layer 6.

FIG. 27H shows surface relief feature 8 formed by a texturedmicro-relief portion of surface 10. The textured portion may besurrounded by non-textured, flat portions of surface 10 thus creating adistinct, well defined microstructured area of the respective feature 8.The texture may be formed in a random or ordered manner, provided thateach elementary micro-relief feature has a low aspect ratio (low heightto width) so that the respective surface slope angles do not exceed acertain maximum angle which would cause light leakage from waveguide 4through surface relief feature 8. According to one embodiment,considering that the walls of micro-relief features of the surfacetexture may have a variable slope from top to bottom, the walls of eachmicro-relief feature should have a sufficiently low prevailing slope toprovide the above discussed operation. Additionally, each micro-relieffeature should have a generally smooth surface on the scale which iscomparable to the wavelength of light of source 400. This requirement isaimed at minimizing or preventing light scattering and/or reflecting athigher than the prescribed angles with respect to the longitudinal axisof waveguide 4 or the plane of surface 10.

It should be understood that surface relief feature 8 are not limited torecesses or undulation-type surface microstructure but may also beformed by light-deflecting surface protrusions or indentations of theappropriate shape and even by varying the thickness of waveguide 4 alongthe light propagation path, provided the prevailing slope of therespective surface structures is less than the prescribed maximum angle.

By way of example and not limitation, the surface of the each texturedarea of FIG. 27H may include an array of microlenses. According to theprinciples discussed above, each of the microlens should have asufficiently low profile ensuring that the slope of the microlens wallswith respect to surface 10 does not exceed the maximum allowed angularvalue so that system 2 may operate in the manner described in the aboveembodiments.

In FIG. 27I, an alternative configuration of surface relief feature 8 isillustrated which includes a low-aspect-ratio, smooth-walled protrusionin surface 10. Referring to a projection of surface relief feature 8onto a plane parallel to surface 10, such protrusion may have anelongated shape, a round shape or any other suitable shape or outline.In a non-limiting example, a two-dimensional protrusion may be formed bydepositing a micro droplet of UV-curable polymer onto surface 10. Thepolymer should preferably have a low surface tension allowing for thedroplet to obtain the desired low-profile shape before curing.Alternatively, surface 10 of waveguide 4 may be specially treated orcoated to increase its surface tension and obtain the desired surfacewettability.

Referring to FIG. 27I and to the example of forming surface relieffeature by a UV-curable droplet, slope angle α may be associated withthe angle at which the liquid contacts the surface. This contact angleis commonly known as the wetting (or dihedral) angle that a liquiddroplet makes to a solid surface. Accordingly, in the illustratedexample, the wetting angle of the droplet with respect to surface 10should be kept sufficiently low in order to form a low-profile surfacerelief feature 8 which could deflect light by a sufficiently small anglewith respect to its original propagation direction.

It is noted that surface relief features 8 may be arranged in surface 10in a variety of ways. For example, surface relief features 8 may beformed in a parallel array of strips or bands extending perpendicular tolongitudinal axis 200 of waveguide 4. The respective strips or bands canbe made substantially straight. Alternatively, they may have a constantcurvature or even some waviness.

FIG. 28 shows an embodiment of system 2 in which waveguide 4 has a shapeof a rectangular plate or slab and in which surface relief features 8are arranged in a parallel linear array which longitudinal axis isperpendicular to axis 200. FIG. 28 also shows an exemplary configurationof light source 400 which includes a strip of LEDs where each LED isprovided with individual collimating element 440. Each collimatingelement 440 intercepts light emitted by the respective LED andcollimates said light towards axis 200. Collimating element 440 may beconfigured to provide light collimation in at least one plane ordimension. By way of example and not limitation, collimating element 440of FIG. 28 may have a linear geometry and may be characterized by alongitudinal axis and an optical plane both of which extendingperpendicular to the plane of the drawing. It will be appreciated thatsuch linear collimating element 440 will provide light collimation in aplane which is parallel to the respective plate or slab. In a furthernon-limiting example, collimating element 440 may have a round apertureand may be configured to collimate light in two orthogonal dimensionssuch as those parallel and perpendicular to the prevailing plane of therectangular plate or slab.

FIG. 29 shows an alternative exemplary arrangement of surface relieffeatures 8 across surface 10 and also shows an alternative configurationof a plurality of collimating elements 440. Referring to FIG. 29 , eachsurface relief feature 8 is formed by a textured area as explained inrelation to FIG. 27H and is separated from the adjacent surface relieffeatures 8 by a spacing area in an ordered array pattern. The size ofeach surface relief feature 8 may be varied. Particularly, the size mayincrease with the distance from light source 400 to compensate the lightdepletion in waveguide 4 along axis 200. Alternatively, the density ofsurface relief features 8 in the array may increase with the distancefrom source 400 for the same reason. Referring further to FIG. 29 , anarray of light collimating elements 440 is arranged on a singletransparent substrate and form a solid piece structure which can beplaced between the LED strip and waveguide 4. Accordingly, lightcollimating elements 440 of FIG. 29 may also be configured to emit lightalong axis 200 and provide light collimation in one or two dimensions.

FIG. 30 depicts an embodiment of illumination system 2 in which bufferlayer 6 is provided on the microstructured surface 10 of waveguide 4.Light extracting layer 20 provided on the external surface of the bufferlayer 6, accordingly so that surface 12 is exposed to the outsidemedium. Layer 6 should provide a good optical contact between waveguide4 and light extracting layer 20. It may be formed as a conformal thincoating (conforming to the shape of surface relief features 8) of aconstant thickness. Alternatively, as shown in FIG. 30 , layer 6 can bemade of a sufficient thickness to fill the depressions in surface 10 andprovide a smooth external surface. The light extracting layer 20 of FIG.30 can be configured to provide strong backscattering of light emergingfrom waveguide 4 with a highly diffuse reflectivity in which case system2 can be advantageously used, for example, for general lightingapplications. Layer 20 may also be associated with an image print inwhich configuration the system 2 can be used, for example, as a frontlight, decorative light or as an illumination cover for signage orpainting/printing arts.

In operation, referring to FIG. 30 , ray 72 propagating in waveguide 4strikes surface 10 where it is reflected from one of surface relieffeatures 8 by means of TIR due to the relatively low propagation anglewith respect to axis 200. The slope of face 16 with respect to theprevailing plane of surface 10 is sufficiently low so as to result inthe incremental increase in the out-of-plane angle of ray 74 withoutcausing it to exit through surface 10. As a result of TIR from face 16,ray 72 obtains slightly lower incidence angle with respect to normal 800and further propagates towards the opposing surface 12 of waveguide 4.

Since the drop in refractive index outwardly from waveguide 4 at itssurface 12 is greater than that at surface 10, the incidence angle ofray 72 into surface 12 is not sufficient to overcome TIR at thatsurface. Therefore, ray 72 continues to propagate in the waveguide modethrough the core of waveguide 4.

In contrast, rays 74 and 76, deflected by the preceding surface relieffeatures 8 (not shown), strike surface 10 at smaller angles with respectto normal 800, said angles being less than the critical angle of TIR atthe interface between waveguide 4 and low-n layer 6. As a result, rays74 and 76 exit from the core of waveguide 4 and strike layer 20 whichscatters the extracted rays back towards the viewer's eye 660.

FIG. 31 depicts an embodiment similar to that of FIG. 30 except thatlight extracting layer 20 is configured to transmit and diffuse lightemerging from waveguide 4 towards the opposing side. In suchconfiguration, system 2 can be used, for example, for backlights,transmissive sign displays or area illumination.

Referring to FIG. 30 and FIG. 31 , exemplary rays 74 and 76 illustratelight emerging from waveguide 4 and illuminating layer 20. As it isfurther illustrated, the emerging light can be further directed by layer20 to provide either front light or back light operation depending onthe configuration of the light scattering/diffusing layer. Furthermore,the surface of layer 20 can be illuminated from one or more additionaldirections and by one or more additional light sources such as, LEDs,lighting luminaires or sunlight. By way of example and not limitation,when layer 20 comprised a transmissive light diffuser, it can beilluminated from the appropriate edge of waveguide 4 by a strip of LEDsand by a beam of sunlight incident onto layer 20 perpendicular to itssurface, in which case system 2 of FIG. 31 may have operation similar toFIG. 25

Referring to FIG. 31 , a distant light source 890 is illustrated whichilluminates system 2 perpendicularly to its plane. Ray 892 passesthrough the transparent layers of waveguide 4 and buffer layer 6 afterwhich it strikes layer 20. In turn, layer 20 scatters ray 892 similarlyto rays 74 and 76, all of which are extracted from the waveguide towarda viewer 600. When light source 890 is exemplified by sunlight, such asthat delivered from a skylight or a similar device, system 2 can be usedas a luminaire for hybrid lighting and can provide combination ofnatural and artificial illumination. In this case, light from source 400can be advantageously made dimmable in response to the change in thesunlight intensity so that constant lighting in the room can bemaintained. The corresponding layers of system 2 may be configured toprovide illumination pattern which is generally symmetrical with respectto no normal 800.

FIG. 32 depicts an embodiment of system 2 in which light extractinglayer 20 is made from an optically transmissive material and includesV-shaped prismatic grooves 380 which slope is selected to redirect lightemerging from waveguide 4 into a perpendicular direction with respect tothe prevailing propagation path of light in the waveguide. It ispreferred that a refractive index n₆ of layer 20 is at least equal orgreater than that of buffer layer 6 to prevent TIR at the boundarybetween the two layers.

Accordingly, rays 74 and 76 emerging first into buffer layer 6 enterlayer 20 and strike respective sloped faces of grooves 380 where saidrays are reflected by beans of TIR towards along a normal to theprevailing plane of waveguide 4. Ray 78 strikes an interface with theair pocket formed by groove 380 and is reflected by TIR back intowaveguide 4 and towards surface 10. However, since surface 10 representsa boundary with the outside medium (air) which has substantially lowerindex than waveguide 4, ray 78 will eventually reflect from surface 10and can thus have a further chance of being fully extracted from system2 by other grooves 380 along the propagation path.

It will be appreciated that the arrangement of FIG. 32 can be configuredto efficiently collimate light at least in the plane of the drawing thatis a plane which is parallel to longitudinal axis 200 of waveguide 4 andwhich is also perpendicular to the prevailing plane of waveguide 4.Accordingly, when waveguide 4 of system 2 is illuminated by anon-collimated or weakly collimated beam of light at its light inputedge or end, the illustrated embodiment of system 2 can emanate highlycollimated light at least in the above-mentioned plane. It will also beappreciated that the degree of such collimation can be adjusted in abroad range, for example, by adjusting the slopes of faces 16 offeatures 8 and/or the slopes or configuration of grooves 380. Moreover,additional angular distribution effects for the emitted light can beachieved by varying the respective slopes of faces 16 and/or grooves 380along the longitudinal axis 200 of waveguide 4.

I will be further appreciated that system 2 may include variouscollimated elements attached to the input edge or end of waveguide 4which can provide additional means for controlling the angular spread ordistribution of light emitted from the respective broad surface ofsystem 2. Particularly, when system 2 employs a planar configuration ofwaveguide 4 and discrete light sources such as LEDs, collimatingelements 440 such as those illustrated in FIG. 28 and FIG. 29 may beused to additionally collimate the incident beam in a plane parallel toaxis 200 and also parallel to the prevailing plane of waveguide 4. Inthis case, system 2 can emit light which is collimated in two orthogonalplanes or dimensions rather than just in a single plane or dimension.Additionally, it is noted that the degree of collimation and angulardistribution of light emitted by system 2 may be independentlycontrolled in each of the orthogonal planes or dimensions thus providinga virtually unlimited number of combinations for the emitted beamparameters. In a non-limiting example, when used as an area illuminationdevice such as ceiling-mounted panel luminaire, system 2 may beconfigured to illuminate a well defined area below the luminaire whileemitting much fewer or no light towards non-functional directions.

According to one embodiment, slope angle α of each face 16 may belimited to about three angular degrees in order to maximize thecollimation power of system 2 and/or minimize the light leakage throughsurface relief features 8. More particularly, according to certainembodiments, slope angle α of each face 16 may be less than two and ahalf degrees, less than two degrees, less than one and a half degrees,and less than one degree. According to one embodiment, the minimum slopeangle α of each face 16 may be about one half of a degree.

The slope of the reflecting walls of grooves 380 may be selected toredirect light emerging from waveguide 4 at any suitable angle withrespect to a normal to the prevailing plane or longitudinal axis ofsystem 2. Particularly, grooves 380 of light extraction layer 20 may beconfigured to redirect the emergent light so as to result in system 2generally emitting light at an off-normal angle from its majorbroad-area surface. Such an off-normal angle may take particular valuesof, for example, thirty degrees, forty five degrees or sixty degrees.Considering that system 2 may be configured to emit a major portion ofcollimated light in a limited range of angles with respect to thesurface normal, such range may be between zero and thirty degrees,between fifteen degrees and forty five degrees, between thirty degreesand sixty degrees, for example. Such off-normal illumination may beuseful for a number of applications of system 2. By way of example andnot limitation, system 2 emitting off-normal collimated beam may be usedto illuminate a portion of a wall from a planar lighting fixture mountedflush with a ceiling, in an application like accent illumination ofwall-mounted fine art drawings and the like.

It should be understood that grooves 380 may be substituted by any otherfeatures capable of redirecting light emerging from waveguide 4 toward aprescribed direction. FIG. 33 shows another non-limiting example oflight redirecting features associated with layer 20 and exemplified bysharp-angle slits 382 in the surface of layer 20. Each slit 382 has arelatively narrow base and extends deep into the body of layer 20 at anangle with respect to surface normal. Such configuration of lightredirecting features may be more advantageous from the point of view ofenhancing the light extraction efficiency. As it can be seen from FIG.33 , ray 78 is extracted from system 2 in the first pass as opposing tothe configuration illustrated in FIG. 32 . The deep and narrow slits 382may be made by any suitable means, including but not limited to laserablation, slitting with a sharp blade, material cracking, etc. Theprocess of forming of such slits should preferably allow for formingoptically smooth walls which can efficiently reflect light by means ofTIR. It is noted that, while straight profiles of the slit walls areillustrated in FIG. 33 , such walls may also have any curvilinear orstepped profile as well.

In a further non-limiting example, slits 382 may be replaced by narrowundercuts made in a surface of layer 20. Each undercut may have parallelor nearly-parallel walls. and may be formed, for instance, by laserablation, a sharp blade or by any other means of material removal orcutting. Similarly, to the above example of slits 382, at least thelight-redirecting wall of each undercut should preferably have anoptically smooth surface allowing for TIR. The light-redirecting wall ofeach undercut should make a dihedral angle with the surface of layer 20which is suitable for light extraction towards a normal direction withrespect to the prevailing plane of system 2.

FIG. 34 illustrates an embodiment of system 2 in which surface relieffeatures have a variable slope with respect to the prevailing plane ofwaveguide 4. Accordingly, system 2 of FIG. 34 includes waveguide 4defined by opposing TIR surfaces 10 and 12, low-index buffer layer 6attached to surface 12, light extraction layer 20 attached to the bufferlayer, and light source 400 coupled to an edge or terminal end of thewaveguide. Surface 10 is provided with an array of micro-stepped surfacerelief features 8. Each feature 8 has a TIR face facing the light sourceand inclined at a sufficiently low angle with respect to the prevailingplane of surface 10 and/or waveguide 4 so that light leaks out ofwaveguide 4 primarily through buffer layer 6 and light extraction layer20. The slope of individual TIR faces increases along the lightpropagation path as a function of the distance from light source 400.This can be useful, for example for compensating of light depletion bythe previous features 8 along the waveguide and for providing a uniformlight output from the light emitting surface of layer 20. According toone embodiment, the slope angle of the TIR faces varies betweenapproximately one half of a degree and three degree. According to oneembodiment, the slope angle of the TIR faces increase with the distancefrom source 400 linearly. According to one embodiment, the slope angleof the TIR faces increase with the distance from source 400 according toa non-linear function in which the rate of slope increase accelerateswith the distance from source 400.

It will be appreciated that system 2 of FIG. 33 is designed to receivelight from one side and emit it from the respective broad-area surface.Such configuration exhibits certain geometrical asymmetry with respectto a normal to the prevailing plane of the system, which, in turn, mypotentially result in an unwanted asymmetry in light distribution in theprevailing plane of light extraction and collimation. In order toeliminate such asymmetry, system 2 may be configured in a symmetricalconfiguration which is also illuminated from the opposing sides bysymmetrically disposed light sources.

FIG. 35 shows a two-sided implementation of system 2 which includes twosymmetrical segments along axis 200. Accordingly, waveguide 4 also hastwo symmetrically disposed segments and two opposing light input edgesor ends which are illuminated with the respective light sources 400 and410. Each of the waveguide segments is provided with an array of surfacerelief features 8 formed in surface 10.

Each of surface relief features 8 is represented by a low-profile,linear asymmetric micro-prism protruding from a base at surface 10outwardly from waveguide 4. The tips or ridges of the micro-prisms mayhave sharp corners or they may also be slightly rounded or even havesmall flat portions.

The larger-area facets of the micro-prisms are tilted outwardly from thecentral portion of waveguide 4 and are configured for incrementaldeflection of propagating in a waveguide mode. Each of the larger-areafacets should preferably have an optically smooth, polished surface topreserve TIR at surface 10 and minimize light loss.

The dihedral angle of the larger-area facets of the micro-prisms isvariable along axis 200, increasing along the intended light propagationpath from the opposing edges or ends of waveguide 4 towards themid-portion of waveguide 4. This ensures that the rate of lightdeflection increases as a function of the distance from the respectivelight source and at least partially compensates light depletion inwaveguide 4 by the preceding micro-prisms. In will be appreciated thatthe dihedral angles of the microprisms may be particularly tailored toprovide for a nearly-constant rate of light extraction from the surfaceof waveguide 4 along axis 200. Additionally, the rate of progressivelight extraction along axis 200 by the prismatic surface relief features8 should preferably be sufficient to ensure that substantially all or atleast a substantial portion of light injected into waveguide 4 may beextracted.

Accordingly, the opposing lateral sides of light extracting layer 20 maybe configured to receive light from the respective portions of waveguide4 and further direct the extracted light towards surface normal.

Referring further to FIG. 35 , each of the opposing segments of system 2may be configured to emit a slightly asymmetric beam with respect to anormal to the emitting broad-area surface. However, since the opposingsegments of system 2 are symmetrical and emit beams that mirror eachother with respect to a surface normal, the resulting light distributionrepresenting a superposition of the respective light beams will alsobecome symmetrical with respect to the plane of symmetry of system 2regardless of the distance from the emitting surface.

Accordingly, by varying the angular distribution and directionality ofthe individual beams emitted by the opposing system 2 segments of FIG.35 , the resulting light beam may be focused by pointing the respectivebeams toward converging directions or a target. Alternatively, theindividual beams may also be defocused by pointing them beams towarddiverging directions or away from a certain target.

It will thus be appreciated that the structure and operation of system 2allow for an unprecedented control of the beam emitted from itsbroad-area surface compared to the conventional art employing waveguideillumination systems. In accordance with at least some embodiments andthe principles of light collimation described above in reference, forexample, to FIG. 9 and FIG. 10 , the angular distribution of lightemerging from waveguide 4 can be made very narrow at least in one planeor dimension. In turn, light extraction layer 20 may be configured todirect the narrow-distribution light emerging from waveguide 4 towards aplurality of distinct directions. Such directions may be parallel,convergent or divergent. The divergent or convergent directions may alsofollow a particular angular pattern. Alternatively, the direction oflight emission from the light emitting region of system 2 may berandomized within a limited range thus providing an improved lightdiffusion without sending light into non-functional directions.

In an illustrative example, each of angular distributions 912, 914, 916,and 918 of FIG. 24 may be directed towards a common area located at adistance from system 2 along a normal to axis 200. As a result,particularly when the respective distributions are sufficiently narrow,the intensity of the light beam in the respective area can be madegreater than in the intensity of the surrounding areas. Thus, system 2may be configured to additionally focus light emitted from its surface.

Such focusing can be achieved by the appropriate configuring the lightextraction layer 20. For instance, when layer 20 employs lightredirecting features such as prismatic grooves 380 of FIG. 32 or slits382 of FIG. 33 , the slope of the respective features may be variedaccordingly so as to provide such focusing of the emitted light. In anexemplary case, referring to FIG. 32 , the TIR walls of respectivegrooves 380 redirecting rays 74 and 76 can make slightly differentdihedral angles to the prevailing plane of waveguide 4 so as to resultin said rays converging at a certain distance from system 2.

FIG. 36 shows an embodiment of illumination system 2 in a front lightconfiguration in which there is provided a light-guiding layer formed byplanar waveguide 4 and light extraction layer 20 positioned immediatelyadjacent to the broad-area surface 12 of the waveguide. Waveguide 4 hashigh optical transmissivity of the material and also high transparencyat least in a direction perpendicular to the waveguide's plane. Thewaveguide is configured for transmitting light from edge to edge bymeans of TIR from at least the opposing surfaces 10 and 12 both of whichare made smooth (polished) and highly transparent. The side walls ofwaveguide 4 can also be made smooth and capable of reflecting light bymeans of TIR. Light source 400 is coupled to the light input edge of thewaveguide. The opposing (terminal) edge can be provided with a mirroredsurface to reflect light back into the waveguide and recycle rays thathave not been extracted from the waveguide in a single pass.

Layer 20 comprises a screen containing an image print which is opaquefor the incident light and includes at least one region which has goodlight scattering or reflective properties. The image may contain anytext, graphics, symbols or patterns and can be exemplified, for example,by a front-illuminated display that can be found in the signageindustry.

Waveguide 4 and layer 20 of FIG. 36 should be preferably positioned veryclose to each other, including the case when they are disposed in aphysical contact by at least portions of their surfaces. However, adirect optical contact without any intermediate layer (such as air)should be avoided in order to prevent the suppression of TIR of surface12 and unwanted premature light leakage from waveguide 4.

Waveguide 4 is exemplified by a transparent plastic matrix with aplurality of very fine optically transmissive glass or plastic particleswhich are incorporated into the matrix in very small proportions byvolume and have forward-scattering properties. The forward-scatteringparticles are meant to mean such scattering particles that scatter lightsubstantially in a forward direction and have a very low or negligiblescattering far sideways and in the reverse direction.

The forward-scattering particles are made from an optically transmissivematerial which refractive index differs from that of the main bulk ofwaveguide 4 by a predetermined amount. A minimum difference inrefractive indices is required for the particles to effectively scatterlight rays that propagate sufficiently close to them. Particularly, itis preferred that the refractive index of the particles differs by morethan 0.02 but less than 0.4 from the refractive index of the body ofwaveguide 4. The forward-scattering particles should preferably befinely separated from each other by considerable distances in order toprovide for consistent and predictive light scattering as well asmaintain high visual transparency and transmissivity of the waveguide.

By way of example and not limitation, the core of waveguide 4 can bemade from PMMA (n_(core)=1.49) and the particles can be made frompolystyrene (n_(part)≈1.59) or FEP (n_(part)≈1.34). The particles canhave a simple, single-component structure or they can have a morecomplex morphology and may be composed by a core and a shell made fromdifferent materials. Depending on the other parameters involved, theparticles can have a mean diameter less than, equal to or greater thanthe wavelength of visible light.

The concentration of forward-scattering particles should be sufficientlylow to keep waveguide 4 optically thin along the perpendicular to theprevailing plane of the waveguide. At the same time, the particleconcentration should be high enough to make waveguide 4 optically thickalong longitudinal axis 200 and yet highly transmissive.

The term “optical depth” or “optical thickness” is commonly directed tomean the quantity of light removed from a light beam by scattering orabsorption during its path through a medium. In the context of thisinvention, as applicable to waveguides including non-absorbing,forward-scattering particles, this term can be more narrowly directed tomean the quantity of light that has been perceptibly scattered from theoriginal propagation path of the light beam. The original propagationpath is the path of the light beam in the homogeneous medium of thewaveguide in the absence of the scattering particles.

Particularly, if I₀ is the reference intensity of radiation in ahomogeneous, weakly-absorbing medium of waveguide 4 and I is theobserved intensity of light propagating along the same optical path,then an optical depth τ of the medium in the presence offorward-scattering particles can be defined by the following expression:I=I₀e^(−τ).

In the optically thin case, that is referring to the case of lightpropagating along the perpendicular to the plane of waveguide 4, τ<<1and e^(−τ)26 1−τ, so that the above expression can be simplified asfollows: I=I₀(1−τ). Accordingly, it at least some embodiments of thepresent invention, waveguide 4 can be configured to perceptibly scatterless than 5% of light propagating along the waveguide's normal and morepreferably scatter less than 2% of said light. In other words, thecolumn density of light scattering particles should be low enough sothat τ is less than 0.02-0.05 along the normal to the waveguide'sprevailing plane and so that the high transparency of the waveguide ismaintained.

At the same time, the concentration and light scattering parameters offorward-scattering particles should be selected to ensure that at leasta substantial part of light is removed from waveguide 4 by means ofscattering as such light propagates through the column of material alonglongitudinal axis 200. According one embodiment, it is preferred that atleast approximately half of the light beam input into waveguide 4through an edge is scattered as it propagates towards the opposingterminal edge.

According to one embodiment, it can be preferred that at least 80% orlight is scattered along its longitudinal propagation in waveguide 4,which corresponds to an optically thick case of τ≥1.6.

An important relevant parameter for estimating the requiredconcentration of light scattering particles is an effectivecross-section σ which defines the area around the particle where lightis likely to be scattered. In general, the scattering cross-section isdifferent from the geometrical cross-section of a particle, and itdepends upon the wavelength of light and the permittivity, shape andsize of the particle. Another important parameter defining the type ofscattering and the effective cross-section of dielectric particles ofdiameter d in a refractive medium is the so-called size parameter x:

${x = \frac{\pi{d \cdot n_{med}}}{\lambda}},$

where n_(med) is the refractive index of the refractive medium and λ isthe wavelength of the propagating light.

The general case for an arbitrary value of x is called Mie scattering inthe relevant art. In the case of relatively large particles compared tothe wavelength (x>>1), the total cross-section tends toward a geometriclimit of σ=πd²/4. As the particles become smaller, the forwardscattering diffraction peak can be observed, in which case the crosssection will be become twice the geometric limit.

In the case of very small particles (x<<1), the so called Rayleighlimit, the total cross-section is given by

${\sigma = {2/3\pi d^{2}{x^{4}( \frac{m^{2} - 1}{m^{2} + 1} )}^{2}}},$

where m is the ratio between the refractive index of the particle andthe refractive index of the medium.

Considering light propagating in waveguide 4 having sparsely distributedscattering particles and assuming that the particles do not shadow eachother, the optical depth can be related to the scattering cross-sectionthrough the following expression: τ=NLσ, where L is the propagation pathlength and N is the number density of light scattering particles.

The relationship between an optical depth along the waveguide'slongitudinal axis τ_(∥) and an optical depth perpendicular to theprevailing plane of the waveguide τ_(⊥) defines the relative differencein waveguide transparency between the respective directions. It can alsobe used to define the relationship between the thickness and the lengthof the waveguide for the given scattering efficiency of the particlesand particle concentration.

The scattering particles should be particularly configured to besubstantially invisible to the naked eye from a distance and should notintroduce a perceptible blur or haze to the bodies or images behindwaveguide 4. Additionally, the scattering particles should be configuredto not introduce a substantial glare or loss in image contrast whenwaveguide 4 is lit by source 400 or by any other external light source.

In a non-limiting example, the core of waveguide 4 may include anAcrylite® Endlighten acrylic sheet (e.g. available from CYRO Industries,Rockaway N.J.) which combines visual transparency and light-scatteringproperties. In a further non-limiting example, waveguide 4 may include aPlexiglas® ELiT acrylic sheet. The sheets can be made using extrusion,casting or any other suitable process and can be configured to provide90-92% light transmission along surface normal.

In operation, referring further to FIG. 36 , the light input edge ofwaveguide 4 receives light emanated by source 400 and refracts lightinto the body of waveguide allowing at least a substantial part of theemitted light to propagate in a waveguide mode. Such light injectedthrough the light input edge of waveguide 4 propagates towards theopposing terminal edge by means of transmission and TIR from surfaces 10and 12 until it is incrementally scattered by a the light scatteringparticles to a sufficient angle to overcome TIR at surface 12 and exittowards light extracting layer 20. The image print of layer 20back-scatters the out coupled light into all directions so that at leasta portion of the scattered light can reach the viewer's eye 660. Thus,the observer can clearly see the image print which is brightly lit bythe highly transparent top layer represented by waveguide 4.

FIG. 37 illustrates the structure and operation of the embodiment ofFIG. 36 in further detail by depicting an exemplary path of ray 74emanated by source 400. Waveguide 4 comprises a large plurality offorward-scattering particles 36 made from a transparent refractivematerial that has different refractive index than waveguide 4. Eachscattering particle 36 is configured to provide strong forwardscattering propertied so that each interaction of light ray with suchparticle can change its propagation direction by up to a relativelysmall, predetermined amount. The bend angle introduced by each particle36 should be low enough to generally prevent light exiting fromwaveguide 4 at high angles with respect to the prevailing plane of thewaveguide. Instead, each scattering particle 36 should be configured toprovide incremental deviation of rays along the propagation path andeventually result in light rays exiting from surface 12 at low angleswith respect to the surface, thus providing means for illuminating layer20 by the decoupled light.

Particles 36 are preferably finely distributed through the volume ofwaveguide 4 in a concentration low enough to maintain high transparencyof the waveguide along normal 800. At the same, time, the concentrationshould be sufficient to extract most of the light from waveguide 4 bymeans of forward scattering and by means of eventual communicatingincidence angles greater then TIR at surface 12.

Due to the probabilistic nature of scattering and particle encounter,ray 74 may undergo multiple scattering events until it reaches theincidence angle greater than the TIR angle at surface 12. It will beappreciated that at certain concentrations of light scattering particles36, a portion of light beam may remain non-extracted upon reaching theterminal edge of waveguide 4. Therefore, the terminal edge may beprovided with reflective surface 602 which will reflect the aboveportion of light back into the waveguide and promote a more completelight utilization. In certain cases, employing reflective layer 602 mayalso result in an improved uniformity of light emitted by system 2.

It will be appreciated by those skilled in the art that when waveguide 4is surrounded by air, the smooth waveguide/air interfaces at surfaces 10and 12 may appear functionally identical with respect to lightpropagating by TIR within the waveguide. Accordingly, with the absenceof TIR suppression or at one of the surfaces, the propagating light hasessentially equal probability of exiting through either surface. As aresult, about 50% of light may exit from waveguide 4 withoutilluminating the image print of layer 20. Although the emergence angleof light exiting through surface 10 can be made sufficiently low so asnot to interfere significantly with viewing the illuminated image printfrom a normal direction, the loss of the respective portion of light maybe unwanted for a variety of reasons.

In order to eliminate or at least substantially reduce this light lossand enhance the brightness of the image print, a suitable intermediateoutcoupling layer can be provided between waveguide 4 and layer 20, asillustrated in FIG. 38 .

Referring to FIG. 38 , the intermediate outcoupling layer is exemplifiedby highly transparent buffer layer 6 disposed in a good optical contactwith both waveguide 4 and layer 20. Similarly to at least some of theembodiments discussed above in reference to, for example, FIG. 5 andFIG. 6 , buffer layer 6 has a lower refractive index than waveguide 4.At the same time, the refractive index of layer 6 should be greater thanthe refractive index of the surrounding air or otherwise of the materialadjacent to surface 10.

Buffer layer 6 allows for TIR at surface 12 at sufficiently highincidence angles, that is when the propagation angle is sufficiently lowwith respect to longitudinal axis 200. Similarly to the embodimentsdescribed in reference to FIG. 5 and FIG. 6 , surface 10 can becharacterized by the first TIR angle ϕ_(TIR1) and surface 12 can becharacterized by the second TIR angle ϕ_(TIR2).

Accordingly, when the incidence angle onto surface 12 becomes lower thanϕ_(TIR2) as a result of scattering by particles 36 and incrementalincrease of the out-of-plane angle, light can escape from waveguide 4through the respective surface and illuminate the image surface of layer20. Since the refractive index of air is lower than that of layer 6, theTIR angle ϕ_(TIR1) at surface 10 is lower than the second TIR angleϕ_(TIR2). This results in the prevailing light escape paths throughlayer 6 and no light or at least much less light leaking through surface10.

Referring further to FIG. 38 , system 2 may also include collimatingelement 440 associated with the light input edge of waveguide 4.Collimating element 440 may be attached to the light input edge ofwaveguide 4 with or without good optical contact or it can be spacedapart from the edge by a small air gap or by an intermediate opticallytransmissive layer. For maximizing the light input, collimating element440 may be coupled to the waveguide edge using an optical adhesive orencapsulant. Collimating element 440 may be configured to narrow theangular light distribution emitted by light source 400. This may ensurethat the second TIR angle ϕ_(TIR2) is not exceeded in waveguide 4prematurely for at least some rays and that no substantial portion oflight exits from the waveguide 4 near the light input edge. Since theembodiment of FIG. 38 may be configured to admit up to about twice asmuch light onto the print of layer 20 and eliminate or at leastsubstantially suppress light loss through the opposing surface 10compared to the embodiment of FIG. 37 , it may be advantageouslyselected for at least some applications where more complete lightutilization and improved image contrast are important.

FIG. 39 depicts an embodiment of system 2 configured as a backlight andmore particularly as a two-sided backlight. An additional pair of thebuffer layer and the light extraction layer is provided on surface 10,as indicated by reference numerals 506 and 520, respectively.

Similarly, the layers 506 and 520 are laminated to surface 10 with agood optical contact with or without intermediate adhesive layers. Bothlight extraction layers 20 and 520 may be configured as transmissivediffusers and can have any suitable color, tint, light transmissive andscattering characteristics. Either one of the layers 20 and 520 may alsobe configured to display an images or any suitable pattern or text.

Accordingly, light propagating through waveguide 4 and scattered byparticles 36 can be evenly distributed along the surfaces 10 and 12 andwill illuminate both layers 20 and 520 thus providing the desiredoperation of a two-sided backlight having waveguide 4 sandwiched betweenthe opposing light extracting layers. Ray 74 of FIG. 39 exemplifieslight exiting from waveguide 4 through surface 12 and ray 76 exemplifieslight emerging from surface 10.

FIG. 40 illustrates the effect of adding layer 6 between waveguide 4 andlight extraction layer 20 for an exemplary case of an isotropicscattering particle 536 embedded into the material of the waveguide. Aray 720 exemplifies light propagating in a waveguide mode throughwaveguide 4.

Referring to FIG. 40 , ray 720 is scattered into all directions by theparticle 536. While surfaces 10 and 12 receive an about equal number ofscattered rays, the low-n layer 6 enables the asymmetry in TIR anglesbetween surfaces 10 and 12 and results in more light extracted throughsurface 12 towards layer 20 compared to the amount of light escapingfrom waveguide 4 through surface 10.

FIG. 41 illustrates the advantage of employing anisotropic scatteringparticles by example of a single forward-scattering particle 36operating in conjunction with the low-n layer 6. As shown in FIG. 41 ,ray 720 is forward-scattered by particle 36 at scattering angles notexceeding the complementary angle to the first TIR angle ϕ_(TIR1). Atthe same time, the maximum scattering angle is greater than thecomplementary angle to the second TIR angle ϕ_(TIR2) which causes atleast some uttermost scattered rays to obtain sub-TIR incidence angleswith respect to surface 12 and exit from waveguide into layer 6. Sincelayer 6 is optically transmissive, all light rays exiting into layer 6reach light extraction layer 20 which, in turn may redirect orredistribute said rays and direct them into the prescribed directions.Accordingly, by limiting the scattering angle by a predetermined value,the unwanted light escape through surface 10 can be eliminated or atleast substantially reduced compared to the case of FIG. 40 whereisotropic particle 536 is employed.

A scattering angle δ can be defined as an angle between the originalpropagation direction that a light ray has before encounteringscattering particle 36 and the direction of a scattered ray resultingfrom ray interaction with the particle. Accordingly, referring to FIG.42 , if ray 720 has out-of-plane propagation angle β before scattering,it may obtain a new out-of-plane propagation angle β+δ thus broadeningthe angular distribution of light propagating in waveguide 4.

In an operational aspect of this invention, forward-scattering particles36 may be configured to provide function similar to surface relieffeatures 8 of at least some embodiments discussed above (see, e.g., thediscussion on broadening the angular distribution of propagating lightbeam in reference to FIG. 7 and FIG. 8 ). More particularly, it may beappreciated that an analogy may be drawn between the scattering angle δof FIG. 42 and the angular increment 2α of FIG. 8 . Although angle δ ofFIG. 42 is obtained by using a light scattering mechanism and aspecially configured particle 36 embedded into the bulk of waveguide 4material while angle 2α is obtained using surface relief feature 8having a characteristic slope α with respect to the waveguide surface,both of these angular values represent a relatively small incrementaldeviation of the respective light rays from the original propagationdirections. Either one of the above-compared light deviation mechanismsmay be configured directed to provide a controlled leakage of lightthrough the designated major surface of waveguide 4. Additionally, it isnoted that by selecting the appropriate parameters of surface relieffeatures 8 and forward-scattering particles 36 of the respectiveembodiments, the angular spread and angular/spatial distribution oflight extracted from waveguide 4 can also be controlled, which may beused, for example, for providing a light collimating function of system2.

It will be appreciated by those skilled in the art that, although eachindividual interaction of a light ray with forward-scattering particle36 has a random character, by selecting the material, refractive index,size, concentration and/or other parameters of particles 36, the lightscattering pattern of the particles may be tailored to provide acontrolled magnitude and rate of spreading of the light beam along thepropagation path in waveguide 4. As layer 6 frustrates TIR for at leastthose rays that have incidence angle into surface 12 greater than thecritical TIR angle ϕ_(TIRC), light will primarily exit from waveguide 4toward layer 20 and not toward the opposing side of the waveguide.

By applying Mie scattering calculations to dielectric spheres inrefractive medium and considering an exemplary case of PMMA waveguide 4(n≈1.49) and polystyrene particles (n≈1.59) a scattering phase functioncan be obtained for various particle sizes. The scattering phasefunction, or phase function, gives the angular distribution of scatteredlight intensity at a given wavelength.

FIG. 43 shows a scattering phase function plot for an exemplaryspherical particle 36 having a 5 μm in diameter, calculated according toMie theory for 532 nm wavelength. As it can be seen from an angulardistribution 972 of scattered light intensity, the scattering angle δgenerally does not exceed 5° for the most part of light beam. Moreparticularly, about 90% of scattered light will be deviated from theoriginal propagation path by 5° or less. Therefore, each interaction oflight beam with such a narrowly-scattering particle 36 will broaden theangular distribution by a small increment which can be sufficient foruseful light extraction through surface 12 and the adjacent layer 20 butgenerally insufficient for light escape through opposing surface 10.

FIG. 44 shows a similar plot calculated for 0.9 μm diameter of particle36 where the scattered light intensity has a different angulardistribution 974. The analysis of angular intensity distribution 974indicates that about 90% of the scattered light beam is confined within±17° scattering cone and that 95% of the beam is contained within ±20°scattering cone, respectively. Thus, statistically, scattering angles δwill generally be greater for the case of 0.9 μm diameter of particle 36than for the case a larger, 5 μm particle 36. Accordingly, light rayshaving angular distribution 974 will be extracted from waveguide 4 muchfaster and will travel much shorter distances along axis 200 than lightrays having angular distribution 972. It will be thus appreciated thatthe particle size can be used for controlling the rate of lightextraction and overall light output for a given area of system 2. Thesize of particles 36 therefore represents an important system parameterthat generally needs to be factored into the system design along withthe other parameters discussed above for the respective embodiments.

According to at least some embodiments of the present invention, thesize and other parameters of forward-reflecting particles 36 can beselected to result in the scattering angles δ that generally do notexceed the difference in TIR angles at surfaces 10 and 12. In otherwords, δ<ϕ_(TIR2)−ϕ_(TIR1), where ϕ_(TIR2) is the critical TIR angle atthe interface of waveguide 4 with buffer layer 6 (e.g., surface 12 inFIGS. 38 ) and ϕ_(TIR1) is the critical TIR angle at the interface ofwaveguide 4 with the outside medium (e.g., surface 10 in FIG. 38 ). Thiscan minimize light escape into air through the waveguide surfaceopposing the interface between waveguide 4 and buffer layer 6.

It should be understood that this invention is not limited to the caseswhere system 2 has the shape of a rectangular plate, sheet or film or anelongated rod and may be applied to the case when system 2 has any othersuitable shape. Particularly, system 2 can have any basic geometricform, a free-form or any combination thereof. Additionally, anytwo-dimensional planar shape of system 2 can be bent in any suitable wayto form a three-dimensional shape. This can be used, for example, toprovide a broader illumination pattern or create artistic or decorativeeffects. Similarly, system 2 having an elongated or rod-like geometrycan be bent or formed to create a two-dimensional or three-dimensionalshape.

FIG. 45 depicts an example of system 2 in an axisymmetricalconfiguration where light source 400 is positioned at the central areaof waveguide 4 and surface relief features 8 are represented by nestedradial rings. Waveguide 4 may have an area 290 which is clear and voidof any features 8.

FIG. 46 shows an alternative round configuration of system 2 in whichthere is a central opening in waveguide 4 where a ring of multiple lightsources 400 is attached to the inner edge of the waveguide. Inconfigurations depicted in FIG. 45 and FIG. 46 , light injected intowaveguide 4 in the central area and propagating radially away from thecenter is emitted from the broad surface of the waveguide by thecombined function of surface relief features 8, buffer layer 6 and lightextraction layer 20, according to the principles described above. Inview of the foregoing description of the invention, it should beunderstood that the above-described forward-scattering particles 36 canalso be used in place or in addition to surface relief features 8.

It should be understood that at least some of the configurations ofsystem 2 shown in a cross-section in the foregoing embodiments may alsobe implemented in an axisymmetrical form obtained by the revolution ofthe respective cross-section around an axis. It should also beunderstood that this invention is not limited to the light input throughan edge into waveguide 4, but can also be applied the case where lightcan be input at any suitable location of waveguide 4, including anarbitrary location across surfaces 10 or 12. The light can be input byembedding light source 400 into the envelope of waveguide. Light source400 can be alternatively attached to a broad-area surface of waveguide 4or spaced apart from the body of the waveguide. When source 400 isexternally attached to waveguide 4, a suitable collimating ornon-collimating light coupler can be used to inject light intowaveguide's core.

At least some of the foregoing embodiments were described upon the casewhere a difference between the refractive indices at the opposing wallsor surfaces of waveguide 4 was used to force light escape towards adesignated side of the waveguide. However, this invention is not limitedto this and may be applied to the case when any other suitable means areused for suppressing light leakage through the unwanted side of thewaveguide. Particularly, a specularly reflective layer, such as amirrored surface, may be provided on the side of the waveguide opposingto the light emitting side to return any stray light back to thewaveguide.

FIG. 47 shown an embodiment of system 2 comprising planar waveguide 4sandwiched between a light turning film 620 and a sheet-form specularreflector 630. Waveguide 4 has generally planar opposing broad-areasurfaces 10 and 12. Surface 12 has a plurality of shallow surface relieffeatures 8 represented by shallow steps formed in said surface.

Light source 400, which may be represented by one or more LEDs, coldcathode fluorescent lamp (CCFL) or any other source, is positioned nearthe light input edge of waveguide 4. Collimating element is provided tocollect light propagating from source 400 away from the light input edgeand inject such light into the waveguide.

Each surface relief feature has a planar face 16 which is configured toreflect light propagating at relatively low out-of-plane angles by meansof TIR, as illustrated by a. The slope angle α of the respective faces16 is sufficiently low so as to generally result in multipleinteractions of light rays before they can obtain the sufficientout-of-plane angle to overcome TIR at either surface 10 or 12. This isillustrated by the example of a light path of a ray 372 in FIG. 47 . Theslope of face 16 of the respective surface relief feature 8 isinsufficient to communicate a sub-TIR incidence angle with respect toeither surface 10 or 12. Accordingly, ray 372 can continue propagatingin the waveguide mode until its interactions with the subsequent surfacerelief features 8 result in reaching the minimum out-of-plane angle tosuppress TIR.

Reflector 630 is positioned adjacent to surface 10 in such a way thatthere is at least minimal air gap between the two. Reflector 630 has ahigh specular reflectivity and is configured to reflect light emergingfrom surface 10 at high exit angles with respect to surface normal backtowards waveguide 4.

Light turning film 620 has a prismatic structure facing waveguide 4. Thegrooves of the prismatic structure are aligned parallel to each other inan linear array which longitudinal axis extends generally perpendicularto longitudinal axis 200 of waveguide 4.

By way of example and not limitations, light turning film 620 may beexemplified by the Transmissive Right Angle Film (TRAF) which iscommercially available from 3M. The TRAF film has a polyester backingsubstrate with the prismatic structure made from modified acrylic resin.It has the acceptance angle of 0° to 20° with respect to the plane ofthe film and a nominal thickness of about 155 μm. Accordingly, surfacerelief features 8 may be configured to gradually increase theout-of-plane angle of light propagating in the waveguide mode andeventually result in light exiting from the waveguide core towards TRAFat such an out-of-plane angle which will be within the acceptance angleof the TRAF. In turn, TRAF may intercept and further redirect theemerging light away from the emitting surface by an angle ofapproximately 70° thus resulting in light emission from the broad-areasurface of system 2 in a perpendicular direction.

It will be appreciated that surface relief features 8 may be configuredto match any other acceptance angle of light turning film 620, simply byadjusting slope angle a of the respective faces 16. Particularly, eachface 16 may have such a slope angle α which will ensure that most lightwill emerge from waveguide 4 also at an out-of-plane angle which iswithin the acceptance angle of film 620.

It will be appreciated that, in order to achieve the desired operation,slope angle α will typically be much lower than the acceptance anglewaveguide 4. Most light rays propagating in waveguide 4 will requiremultiple interactions with surface relief features 8 along thepropagation path in order to obtain the minimum out-of-plane angle toovercome TIR at surface 12 and yet to enter film 620 at the prescribedlow out-of-plane angles. It will thus be appreciated that most lightwill be extracted from waveguide 4 by deviating from the originalpropagation path in an incremental fashion and that the increments tothe out-of-plane angle communicated by each surface relief feature 8will be relatively low due to the smallness of slope angle α.

A light ray 374 of FIG. 47 exemplifies the final portion of the lightpath according to the above scenario. Ray 374 emitted by source 400 andinjected into waveguide 4 has a relatively high out-of-plane angle withrespect to the waveguide's prevailing plane as a result of its multipleinteractions with surface relief features 8 along the propagation path.Particularly, the angle which is complementary to the out-of-plane angleof ray 374 is lower than the critical TIR angle at surface 12 by arelatively small difference which may be overcome by an additionalinteraction with a single surface relief feature 8. As ray 374 strikesface 16 of the respective surface relief feature 8, it is reflected bymeans of TIR and obtains an increment in the out-of-plane angle. It willbe appreciated by those skilled in the art that the increment in theout-of-plane angle will be twice the slope angle α of face 16.

Accordingly, as ray 374 is directed towards the opposing surface 12, itsnew incidence angle with respect to a normal to surface 12 may becomeless than the critical

TIR angle at that surface resulting in ray decoupling from waveguide 4.Upon exiting from a greater-index material of waveguide 4 into a low-noutside medium (such as air), ray 374 will bend substantially towardsthe plane of waveguide 4 due to the refraction angle being greater thanthe angle of incidence. Therefore, even if ray 374 was making arelatively high out-of-plane angle when propagating within waveguide 4,its respective out-of-plane angle outside of waveguide 4 would generallybe substantially lower, in accordance with the Snell's law, includingnear-zero angles. Accordingly, the maximum allowable angle that ray 374can make with respect to surface 12 can be set to the acceptance angleof film 620 in which case substantially all rays emerging from waveguide4 may be turned towards a perpendicular direction. Since film 620 may beconfigured to generally preserve the angular distribution of light thatit redirects and since most rays emerging from waveguide 4 may bedistributed within a relatively narrow angular cone, a highly collimatedbeam may thus be obtained.

Referring further to FIG. 47 , the maximum allowable slope angle α_(MAX)of face 16 may be defined from the following reasoning for a givenacceptance angle ϕ_(ACC) of light turning film 620. It is noted thatthis reasoning is provided by way of an illustrative example to assistthe reader in understanding of the operation of this invention andshould not be construed as limiting the scope of the invention.

Since the out-of-plane angle of ray 374 should preferably be equal to orlower than the acceptance angle ϕ_(ACC) upon exiting from waveguide 4,the minimum refraction angle ϕ_(RMIN) of ray 374 counted off from anormal to surface 12 should be complementary to angle ϕ_(ACC), that isϕ_(RMIN)=π°−ϕ_(ACC). By using the Snell's law, it can be shown that theminimum angle of incidence ϕ_(IMIN) corresponding to ϕ_(RMIN) can befound from the following relationship: n₁ sin ϕ_(IMIN)=n₀sin(90°−ϕ_(ACC)) or n₁ sin ϕ_(IMIN)=n₀ cos (ϕ_(ACC)) , where n₁ is therefractive index of the material of waveguide 4 and n₀ is the refractiveindex of the outside medium. Accordingly, assuming the outside mediumbeing the air with n₀≈1, sin ϕ_(IMIN)=cos(ϕ_(ACC))/n₁.

Since the angle of reflection is equal to the angle of incidence inrespect to a normal to face 16, TIR from face 16 will decrement theincidence angle or ray onto surface 12 by 2α. Now, considering that theminimum incidence angle that ray 374 may take with respect to a normalto surface 12 while propagating in a waveguide mode is the critical TIRangle ϕ_(TIRC) and that the incidence angle of reflected ray should notexceed ϕ_(IMIN), obtain α_(MAX)=(ϕ_(TIRC)−ϕ_(IMIN))/2.

For the case illustrated in FIG. 47 and in view of the aboveassumptions, ϕ_(TIRC)=arcsin(n₀/n₁). Accordingly, slope angle α of face16 may be selected to not exceed α_(MAX), where

α_(MAX)=1/2(arcsin(1/n ₁)−arcsin(cos(ϕ_(ACC))/n ₁))

For instance, if waveguide 4 of FIG. 47 is made from PMMA (acrylics)having refractive index n₁≈1.49 and light turning film 620 includes TRAFhaving the acceptance angle ϕ_(ACC) of 20°, it will give α_(MAX)≈1.5°.It is noted however, that greater slope angles may also be selected forfaces 16 in which case at least some rays may emerge from waveguide 4 atout-of-plane angles beyond the acceptance angle of film 620.Accordingly, a portion of light emitted from system 2 may have aprescribed degree of collimation and another portion of the emittedlight may have a more diffuse angular distribution. It will beappreciated that, when the primary function of system 2 of FIG. 47 is toemit a collimated light beam while minimizing light scattering, slopeangle α shall either not exceed the maximum angle α_(MAX) or exceed itonly by a relatively small amount. Particularly, the slope angle α ofeach face 16 with respect to prevailing plane of waveguide 4 may be madeless than two angular degrees. It can be shown that, at such low angle,at least a major portion of light escaping through surface 12 will stillhave out-of-plane angles less than 20 degrees, which is within theacceptance angle of the TRAF film. Accordingly, when film 620 isexemplified by the TRAF film, practically all of the emerging light maybe redirected towards the surface normal while generally preserving thenarrow, 20-degrees angular spread and resulting in system 2 emission ofa collimated beam perpendicularly to its plane.

Light turning film 620 may also be provided with light scatteringproperties or associated with an external light scattering layer. Thismay be useful, for example, for smoothing out the irregularities inlight distribution that may be present in the collimated beam. Inanother example, the light scattering features associated with lightturning film 620 may also be useful for additional spreading of thecollimated beam emitted by system 2 over a broader angular range.

At least some of the foregoing embodiments were described upon the casewhere surface relief features 8 are arranged in parallel rows in asurface of waveguide 4. However, this invention is not limited to thisand may be applied to the case where surface relief features arearranged in an array which can have any other configuration. Forexample, as illustrated in FIG. 48 , surface relief features may beformed in surface 10 in an annular stepped arrangement radiallyextending from source 400 and being symmetrical with respect to axis200. However, it is noted that any other suitable pattern may also beused for arranging feature 8 in surface 10, which may also includesymmetric, asymmetric, intermittent, ordered or random patterns. Forinstance, any of the surface distribution patterns of light extractingfeatures of the prior art devices, also including those shown in FIG. 1through FIG. 3 , may be used to distribute surface relief features 8across a major surface of waveguide 4.

The foregoing embodiments were described upon the case where the lightdeflecting elements are exemplified by either surface relief features 8formed in a major surface of waveguide 4 or forward-scattering particles36 distributed though the body of the waveguide. However, this inventionis not limited to this and may be applied to the case where the lightdeflecting elements have any other type provided that system 2 has thesame basic operation.

For instance, the light deflecting elements may include diffraction orholographic structures distributed along either one or both of surfaces10 and 12. Such structures may be configured to deflect light by smallangles in an incremental fashion along the propagation path in waveguide4 and to cause controlled light leakage from the waveguide at relativelylow out-of-plane angles.

In a further instance, the light deflecting elements that can providethe incremental light deflection along the optical path in waveguide 4may also be formed by a corrugated boundary between two differenttransmissive materials incorporated into the body of the waveguide. Anillustrative example of such waveguide is shown in FIG. 49. Accordingly,waveguide 4 of FIG. 49 has a planar slab shape and is formed by twooptically transmissive dielectric layers 642 and 644 disposed in opticalcontact with each other. The refractive index of layer 642 is n₁ and therefractive index of layer 644 is n₃₁ which is different than n₁. Thedifference in the refractive indices creates a refractive opticalinterface between layers 642 and 644 which bends all rays entering it atany non-zero incidence angles. In the case illustrated in FIG. 49 ,n₃₁<n₁.

The boundary between layers 642 and 644 is not planar and includesprismatic corrugations each formed by a pair of planar facets 670 and672. The corrugations may be adjacent to each other and form acontinuous corrugated boundary between the respective layers. It isnoted, however, that the corrugations may also be alternated with flatportions of the boundary and may also be distributed along axis 200according to any suitable pattern. Facet 670 makes a dihedral angle 766with the prevailing plane of waveguide 4 and facet 673 makes a dihedralangle 768 with the same plane. According to one embodiment, angle 766 isgreater than angle 768 and may take particular values up to 90 degrees.According to one embodiment, angles 766 and 768 may be identical ornearly identical. According to one embodiment, angle 766 may be lessthan angle 768.

In operation, a ray 722 propagating in layer 642 of waveguide 4 at anout-of-plane angle β strikes facet 670 of the inter-layer boundary andrefracts into the lower-index layer 644. Ray 722 further strikes surface12 of waveguide 4 and reflects from the surface by means of TIR. Ray 722further refracts at facet 672 and enters layer 642 again. When dihedralangle 768 of facet 672 is lower than the dihedral angle 766 of facet670, the consecutive refraction of ray 722 at facets 670 and 672 willresult in a new out-of-plane propagation angle β+ω which. It will beappreciated that the angular increment co to the out-of-plane angle is afunction of the difference in refractive indices between layers 642 and644 and the difference between dihedral angles 766 and 768. Accordingly,it will also be appreciated that such waveguide 4 with a corrugatedinter-layer boundary may be configured to provide an incrementalincrease of the out-of-plane angle along the propagation path. In turn,this may eventually result in ray 722 exiting from waveguide 4 and beingintercepted and redirected by the light extraction layer (not shown)according to the principles described above.

Various surface treatment techniques other than microstructuring orreplication may also be used to produce suitable light deflectingelements which could incrementally bend light in small angularincrements along the propagation path. For example selective UV exposureor chemical processing may be used to cause repetitive variations in therefractive index along the waveguide's surface or through the body ofthe waveguide. The variations of the refractive index along thepropagation path may create a number of optical interfaces which canbend light by a relatively small angle upon each interaction with thepropagated light. As light rays accumulate a sufficient increment to theout-of-plane angle to overcome TIR, they can exit from waveguide 4 wherethey can be further redirected by light extracting layer 20.

System 2 may also incorporate any number of auxiliary layers servingvarious purposes, such as, for example, providing additional mechanicalstrength, environmental resistance, peel resistance, improved visualappearance, color, etc. Any optical interface between a layer formed bya lower refractive index transmissive medium and a layer formed by ahigher refractive index transmissive medium may also be provided with anintermediate optically transmissive layer, for example, for promotingthe optical contact or adhesion between the layers. The intermediatelayer should preferably have a refractive index which is greater thanthe lower of the two refractive indices at the given optical interface.

It may be appreciated that the system 2 may be implemented in a sheetform and may provide efficient light distribution across a large-areaand emission of a collimated beam from its entire surface, which mayfind utility in various lighting devices. Particularly, system 2 may beemployed for making directional lighting luminaires and fixtures havinga wide emission area, compact form and reduced glare.

FIG. 50 shows an exemplary embodiment of a low-profile directionalluminaire 1050 which incorporates system 2 including at least edge-litplanar slab waveguide 4 and light extraction layer 20 configured forlight collimation. Such luminaire 1050 may be configured in the form ofa panel installable in a similar manner to the fluorescent troffers ornon-collimating direct-lit and edge-lit LED lighting panels. By way of anon-limiting example, luminaire 1050 may be designed for drop inceilings in commercial and institutional buildings and have dimensionscommon for such applications. For instance, similarly to the drop-intiles of the common suspended grid ceilings, the outer dimensions ofluminaire 1050 may be 2′×2′, 2′×4′, or 1′×4′ so that the luminaire maydirectly replace the tile panels in the grid.

Referring further to FIG. 50 luminaire 1050 includes two linear arraysof high-brightness LEDs 878 and 880, respectively, attached to theopposing edges of waveguide 4. Each LED array (or LED strip) may bemounted on a heat sink (indicated as 452 and 454) to dissipate the heatgenerated by LEDs 878 and 880. Each of the heat sinks 452 and 454 may bemade from aluminum in the form of a rectangular block, a hollowrectangular pipe, a channel or any structural profile having theappropriate cross-section. Various conventional finned or finless heatsinks may also be employed.

Each of LEDs 878 and 880 may be provided with some kind of individualcollimating or beam shaping optics. For instance, each LED may include adome shaped lens which can aid in light input into waveguide 4 andreducing light spillage.

Luminaire 1050 may include a metal or plastic housing 462 configured tohold various luminaire components together particularly includingwaveguide 4, light extracting layer 20, LED strips, heat sinks, wiring(not shown), etc. When housing 462 is made from a plastic material, thematerial may be opaque or transmissive/translucent. Housing 462 mayenclose just the perimeter area of system 2 or may also cover thenon-luminous back surface of the system. In either case, especially whenhousing 462 is made from an opaque or poorly transmitting material, itshould have an opening corresponding to the light emitting area ofsystem 2. The dimensions of the opening should preferably be at leastslightly smaller than the dimensions of waveguide 4 to ensure thatsystem 2 and any of its components can be stored securely inside housing462. In other words, an outline 1002 of the waveguide should besufficiently large compared to the opening so that the waveguide won'tfall through the opening under normal operation and use of luminaire1050. Alternatively, or in addition to that, the opening may be coveredby an optically transmissive sheet of plastic material. The transmissivesheet may also be provided with light diffusing properties and may bemade a part of the optical assembly of system 2.

Luminaire 1050 may be provided with any number or light diffusing layersor otherwise beam-shaping layers. For example, an opaque light diffusingsheet of a reflective type may be provided on the back of waveguide 4 torecycle and homogenize stray light. A light diffusing sheet of atransmissive type may be provided on the opposing (light emitting) sideof the waveguide to smooth out possible non-uniformities of the emittedbeam and/or further control beam spread.

Luminaire 1050 of FIG. 50 is also shown to include an LED driver 554representing a self-contained power supply that has outputs matched tothe electrical characteristics of the strips of LEDS 400 and 880. LEDdriver 554 may be attached to housing 462 or it may also be incorporatedinto the housing. Alternatively, LED driver 554 may be located at adistance from the main body of luminaire 1050 and may be electricallyconnected to the LED arrays via a power cord of a suitable length.

LED driver 554 may ordinarily be current-regulated and configured todeliver a consistent current over a range of load voltages. LED driver554 may also be configured to provide dimming of the LEDs by means ofpulse width modulation (PWM) circuits or by any other suitable means.The LED driver may also have more than one channel for separate controlof the opposing LED arrays or for separate control of individual LEDs orLED groups within the arrays. The respective LEDs or LED groups, inturn, may be configured to emit light in different colors or differentcolor temperatures thus allowing for obtaining various static or dynamicillumination effects and/or for just varying the color of light emittedby luminaire 1050.

The structure of system 2 which is incorporated into luminaire 1050 ofFIG. 50 may be selected from any of the foregoing embodiments of anytheir variations. Particularly, by employing a light-collimatingembodiment of system 2 with planar waveguide 4, luminaire 1050 may beconfigured to emit a highly directional light beam into a prescribedangular range or towards a well-defined area to be illuminated.

Depending on the application, the beam angle of luminaire 1050 may belimited to any particular value which is considerably less than a full180-degree in any plane. It will be appreciated that since system 2 mayprovide light collimation at least in a longitudinal plane which isparallel to axis 200 and perpendicular to the prevailing plane ofwaveguide 4, the directionality of such luminaire 1050 can besubstantially enhanced in comparison to the conventional edge-litlighting panels which typically emit light according to a highlydiffuse, lambertian pattern. As it has been illustrated in reference tothe above-described embodiments of system 2, beam collimation may alsobe provided in a transversal plane (a plane perpendicular to axis 200),for example by using and appropriately configuring the collimatingelements which may be associated with individual LEDs (see, e.g., FIG.28 and FIG. 29 ). Accordingly, the edge-lit luminaire 1050 employingsystem 2 and fewer LEDs may be configured to provide highly directional,wide-area beam patterns normally attainable only by using direct-lit LEDpanels having large two-dimensional arrays of discrete light sourceswith individual optics. It is essential that system 2 may be configuredto emit substantially all of the light extracted from waveguide 4 intofunctional directions with minimum or no light rejection.

Considering that, in many practical applications, the light beam emittedby system 2 and/or luminaire 1050 may not have sharply definedboundaries, the beam angle may be defined as two times the verticalangle at which the intensity is 90% of the maximum beam intensity. Inturn, the vertical angle may be defined as an angle between the centerof the emitted directional beam and the direction in which the beamintensity is evaluated. For example, with the luminaire 1050 pointeddownward, a vertical angle of 0° may thus describe the center of adirectional beam emitted along the surface normal.

By way of example and not limitation, the beam angle of directionalluminaire 1050 may be limited at least in one plane to less than 120degrees. By way of further non-limiting examples, the beam angle may belimited to 90 degrees, 75 degrees, 60 degrees, 45 degrees, 30 degrees,or 15 degrees. It is noted, however, that any other practical limits forthe beam angle may be also established depending on the desiredillumination pattern and system 2 can be configured accordingly. It willalso be understood that the beam angle may be controlled independentlyin each of the longitudinal and transversal planes which are orthogonalwith respect to each other. In one embodiment, the beam angle may bemade the same or similar in both planes. In one embodiment, the beamsangles may be made different in the respective planes. In oneembodiment, the collimation may be provided in only one of the aboveplanes. For example, the beam angle in the longitudinal plane may be 60degrees and the beam angle in the transversal plane may be up to a full180 degrees with a diffuse lambertian or gaussian pattern.

Further details of operation of waveguide illumination system 2 shown inthe drawing figures as well as its possible variations and furtherapplications will be apparent from the foregoing description ofpreferred embodiments. Although the description above contains manydetails, these should not be construed as limiting the scope of theinvention but as merely providing illustrations of some of the presentlypreferred embodiments of this invention. Therefore, it will beappreciated that the scope of the present invention fully encompassesother embodiments which may become obvious to those skilled in the art,and that the scope of the present invention is accordingly to be limitedby nothing other than the appended claims, in which reference to anelement in the singular is not intended to mean “one and only one”unless explicitly so stated, but rather “one or more.” All structural,chemical, and functional equivalents to the elements of theabove-described preferred embodiment that are known to those of ordinaryskill in the art are expressly incorporated herein by reference and areintended to be encompassed by the present claims. Moreover, it is notnecessary for a device or method to address each and every problemsought to be solved by the present invention, for it to be encompassedby the present claims. Furthermore, no element, component, or methodstep in the present disclosure is intended to be dedicated to the publicregardless of whether the element, component, or method step isexplicitly recited in the claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. 112, sixth paragraph, unlessthe element is expressly recited using the phrase “means for.”

What is claimed is:
 1. An illumination panel, comprising: an opticallytransmissive sheet having a first broad-area surface and an opposingsecond broad-area surface, the second broad-area surface having atwo-dimensional pattern of light deflecting surface structures; aplurality of dimmable light emitting diodes (LEDs) configured to emitlight towards the optically transmissive sheet, wherein a first group ofLEDs is configured to emit light in a first color temperature and asecond group of LEDs is configured to emit light in a second colortemperature which is different than the first color temperature; and anLED driver electrically connected to the LEDs and comprising a first LEDintensity control channel configured to control a light emissionintensity of the first group of LEDs, a second LED intensity controlchannel configured to control a light emission intensity of the secondgroup of LEDs, and one or more pulse width modulation (PWM) circuitsconfigured to provide dimming of the LEDs in response to a change in anintensity of natural daylight incident onto an area of the illuminationpanel.
 2. The illumination panel as recited in claim 1, wherein theoptically transmissive sheet has a rectangular shape and is configuredto guide light using optical transmission and a total internalreflection, wherein the light deflecting surface structures are discreteelements separated from each other and from edges of the opticallytransmissive sheet by smooth portions of the second broad-area surfaceand each configured for suppressing the total internal reflection at thesecond broad-area surface, wherein the optically transmissive sheet hastwo symmetrical segments each having a light input edge, wherein theplurality of LEDs comprise a first LED strip mounted to a first heatspreading substrate and a second LED strip mounted to a second heatspreading substrate, wherein the first and second LED strips areoptically coupled pairwise to the respective input edges, wherein adensity of the light deflecting surface structures within a first one ofthe two symmetrical segments increases with a distance from the firstLED strip, and wherein a density of the light deflecting surfacestructures within a second one of the two symmetrical segments increaseswith a distance from the second LED strip.
 3. The illumination panel asrecited in claim 2, wherein both the first and second broad-areasurfaces are configured for outputting light from the opticallytransmissive sheet.
 4. The illumination panel as recited in claim 2,comprising an optically transmissive diffuser sheet positioned on afront side of the optically transmissive sheet, a highly reflectivediffuser sheet positioned on a rear side of the optically transmissivesheet, and a metal housing enclosing a perimeter area of the opticallytransmissive sheet and the plurality of LEDs, the metal housing havingan opening located on the front side of the optically transmissivesheet, wherein the opening has a smaller area than the opticallytransmissive sheet.
 5. The illumination panel as recited in claim 4,wherein the illumination panel has outer dimensions of approximately 60centimeters by 60 centimeters, and wherein the LED driver is attached toa back cover of the metal housing.
 6. The illumination panel as recitedin claim 4, wherein the illumination panel has outer dimensions ofapproximately 60 centimeters by 120 centimeters, and wherein the LEDdriver is attached to a back cover of the metal housing.
 7. Theillumination panel as recited in claim 4, wherein the illumination panelhas outer dimensions of approximately 30 centimeters by 120 centimeters,and wherein the LED driver is attached to a back cover of the metalhousing.
 8. The illumination panel as recited in claim 4, wherein theLED driver is connected to the plurality of LEDs using a power cord andconfigured to be positioned at a distance from a main body theillumination panel.
 9. The illumination panel as recited in claim 1,comprising an optically transmissive diffuser sheet positioned on afront side of the optically transmissive sheet, a highly reflectivediffuser sheet positioned on a rear side of the optically transmissivesheet, and a metal housing covering a back side of the illuminationpanel and enclosing a perimeter area of the optically transmissive sheetand the plurality of LEDs, the metal housing having an opening locatedon the front side of the optically transmissive sheet, wherein theopening has a smaller area than the optically transmissive sheet, andwherein the LED driver is attached to a back cover of the metal housing.10. The illumination panel as recited in claim 1, comprising a firstdiffuser sheet positioned on a front side of the optically transmissivesheet, a second diffuser sheet positioned on a rear side of theoptically transmissive sheet, and a metal housing enclosing a perimeterarea of the optically transmissive sheet and the plurality of LEDs, themetal housing having an opening located on the front side of theoptically transmissive sheet, and wherein the opening has a smaller areathan the optically transmissive sheet.
 11. An illumination panel,comprising: an array of dimmable light emitting diodes (LEDs) having afirst plurality of LEDs configured to emit light in a first colortemperature and a second plurality of LEDs configured to emit light in adifferent second color temperature; an optically transmissive sheetdisposed in an energy receiving relationship with respect to the LEDsand having a first broad-area surface and an opposing second broad-areasurface, the second broad-area surface having a two-dimensional patternof light deflecting surface structures; an LED driver electricallyconnected to the array of LEDs; two or more LED intensity controlchannels; and one or more pulse width modulation (PWM) circuits, whereinthe LED driver is configured to provide dimming of the LEDs using theone or more PWM circuits in response to a change of an intensity ofnatural daylight in a space.
 12. The illumination panel as recited inclaim 11, wherein the first and second broad-area surfaces areconfigured for outputting light from the optically transmissive sheet.13. The illumination panel as recited in claim 11, wherein the opticallytransmissive sheet is configured to guide light using opticaltransmission and a total internal reflection, wherein the lightdeflecting surface structures are discrete elements separated from eachother and from edges of the optically transmissive sheet by smoothportions of the second broad-area surface and each configured forsuppressing the total internal reflection at the second broad-areasurface, wherein the optically transmissive sheet has two symmetricalsegments each having a light input edge, wherein the plurality of LEDscomprise a first LED strip mounted to a first heat spreading substrateand a second LED strip mounted to a second heat spreading substrate,wherein the first and second LED strips are optically coupled pairwiseto the respective light input edges, wherein a density of the lightdeflecting surface structures within a first one of the two symmetricalsegments increases with a distance from the first LED strip, and whereina density of the light deflecting surface structures within a second oneof the two symmetrical segments increases with a distance from thesecond LED strip.
 14. The illumination panel as recited in claim 13,comprising an optically transmissive diffuser sheet positioned on afront side of the optically transmissive sheet, a highly reflectivediffuser sheet positioned on a rear side of the optically transmissivesheet, and a metal housing enclosing a perimeter area of the opticallytransmissive sheet and the plurality of LEDs, the metal housing havingan opening located on the front side of the optically transmissivesheet, wherein the opening has a smaller area than the opticallytransmissive sheet.
 15. The illumination panel as recited in claim 14,wherein the illumination panel has outer dimensions of approximately 60centimeters by 60 centimeters, and wherein the LED driver is attached toa back cover of the metal housing.
 16. The illumination panel as recitedin claim 14, wherein the illumination panel has outer dimensions ofapproximately 60 centimeters by 120 centimeters, and wherein the LEDdriver is attached to a back cover of the metal housing.
 17. Theillumination panel as recited in claim 14, wherein the illuminationpanel has outer dimensions of approximately 30 centimeters by 120centimeters, and wherein the LED driver is attached to a back cover ofthe metal housing.
 18. The illumination panel as recited in claim 14,wherein the LED driver is connected to the plurality of LEDs using apower cord and configured to be positioned at a distance from a mainbody of the panel.
 19. The illumination panel as recited in claim 11,comprising an optically transmissive diffuser sheet positioned on afront side of the optically transmissive sheet, a highly reflectivediffuser sheet positioned on a rear side of the optically transmissivesheet, and a metal housing covering a back side of the illuminationpanel and enclosing a perimeter area of the optically transmissive sheetand the plurality of LEDs, the metal housing having an opening locatedon the front side of the optically transmissive sheet, wherein theopening has a smaller area than the optically transmissive sheet, andwherein the LED driver is attached to a back cover of the metal housing.20. The illumination panel as recited in claim 11, comprising a firstdiffuser sheet positioned on a front side of the optically transmissivesheet, a second diffuser sheet positioned on a rear side of theoptically transmissive sheet, and a metal housing enclosing a perimeterarea of the optically transmissive sheet and the plurality of LEDs, themetal housing having an opening located on the front side of theoptically transmissive sheet, and wherein the opening has a smaller areathan the optically transmissive sheet.